Question: The Men's 2013 US Open Series was won by which Spanish player, currently World #1?

full_context: Document 1: Toni Nadal: Antonio Nadal Homar (born 27 February 1961 in Manacor, Mallorca) is a Spanish tennis coach. Toni Nadal is the uncle and coach of tennis player Rafael Nadal and the older brother of Spanish footballer Miguel Ángel Nadal.
Document 2: 2011 US Open Series: In tennis, the eighth edition of the US Open Series (known as Olympus US Open Series for sponsorships reasons), includes ten hard court tournaments that start on July 18, 2011 in Atlanta and will end in Winston-Salem for the men and in New Haven for the women on August 28, 2011. This edition has scheduled five separate men's tournaments, four women's tournaments, and the Western & Southern Open that will host both a men's and women's event. The series includes two ATP World Tour Masters 1000 and two WTA Premier 5 events to headline the series.
Document 3: 2013 Rafael Nadal tennis season: The 2013 Rafael Nadal tennis season officially began on February 5 with the ATP Tournament in Viña del Mar in Chile. He lost in the final, but rebounded with victories at São Paulo and Acapulco. He then won the first Masters 1000 event of the year at Indian Wells. Nadal skipped Miami, and finished Runner up to Novak Djokovic at Monte-Carlo. He followed that with victories at Barcelona, Madrid, and then Rome. At the French Open, he defeated Djokovic in an epic semifinal that went to 9–7 in the fifth set, before thrashing David Ferrer to win his 8th Roland Garros crown. He was then stunned by Steve Darcis in the first round of Wimbledon, in what would be his only defeat prior to the semifinals all season. Limping in the final set, many people thought his season was over, however a strong summer saw Nadal win Montreal, Cincinnati, and then the U.S. Open (def. Djokovic), thus clinching the Emirates Airlines U.S. Open Series. This granted him 3.6 million US dollars in prize money, the most money earned by a tennis player at a single tournament. A few days after the U.S. Open Final, he flew to Madrid to help Spain secure their World Group Playoff spot for 2014, with a victory against Sergiy Stakhovsky. He also scored a victory in doubles with Marc Lopez. These victories gave him a 75–7 (91.46%) record in singles, and 6–1 (85.71%) record in doubles in 2013.
Document 4: Juan Carlos Ferrero: Juan Carlos Ferrero Donat (] ; born 12 February 1980) is a retired professional tennis player, and a former world No. 1 player, from Spain. He captured the men's singles title at the 2003 French Open, and in September of that year, became the 21st player to hold the world No. 1 ranking. He was also the runner-up at the 2002 French Open and the 2003 US Open. His nickname is "Mosquito" due to his speed and his slight physical build. Ferrero retired from the game after the 2012 Valencia Open 500.
Document 5: Djokovic–Nadal rivalry: The Djokovic–Nadal rivalry is between tennis players Novak Djokovic and Rafael Nadal, who are engaged in a modern-day tennis rivalry. The rivalry is the most prolific of the Open Era. The two have faced each other 50 times with Djokovic leading 26–24. Between 2006–2010, this rivalry was often overshadowed by the historic Federer–Nadal rivalry, but it started to become widely recognized after they played their first Grand Slam final against each other at the 2010 US Open. The first half of the rivalry was dominated by Nadal (who led 16–7 after the 2010 season) while the second half has been dominated by Djokovic, winning 11 out the last 13 matches. It is widely considered to be one of the greatest rivalries in tennis history. It was the first rivalry to involve meetings at all 4 Grand Slam finals (including four consecutive Grand Slam finals) and a record 26 Masters Series matches. Their 2012 Australian Open final is considered to be one of the greatest matches of all time, and Mary Carillo and John McEnroe both commented that that Australian Open match and the 2013 French Open semi-final match were the two best matches played on hard and clay courts respectively. The Djokovic–Nadal rivalry was listed as the third greatest rivalry in the last decade in 2009 by "ATPworldtour.com" despite its only starting in 2006.
Document 6: 2016 US Open Series: In tennis, the 2016 US Open Series (known as Emirates Airline US Open Series for sponsorship reasons) is the thirteenth edition of the US Open Series, which includes a group of hard court tournaments that starts on July 18, 2016 in Stanford and concludes in Connecticut for the women and in Winston Salem for the men on August 27, 2016. This edition consists of three separate men's tournaments and three women's tournaments, with the Western & Southern Open hosting both a men's and women's event. The series is headlined by two ATP World Tour Masters 1000 and two WTA Premier 5 events.
Document 7: 2013 Barcelona Open Banco Sabadell: The 2013 Barcelona Open Banco Sabadell (also known as the "Torneo Godó") was a men's tennis tournament played on outdoor clay courts. It was the 61st edition of the event and it was part of the ATP World Tour 500 series of the 2013 ATP World Tour. It took place at the Real Club de Tenis Barcelona in Barcelona, Catalonia, Spain, from 20 April through 28 April 2013. Rafael Nadal won his record 8th title at the tournament.
Document 8: 2013 US Open Series: In tennis, the 2013 US Open Series (known as Emirates Airline US Open Series for sponsorships reasons) was the tenth edition of the US Open Series, which included ten hard court tournaments that started on July 20, 2013 in Atlanta and concluded in Winston-Salem for the men and in New Haven for the women on August 24, 2013. This edition consisted of four separate men's tournaments and four women's tournaments, with the Western & Southern Open hosting both a men's and women's event. The series was headlined by two ATP World Tour Masters 1000 and two WTA Premier 5 events. Rafael Nadal and Serena Williams were the US Open Series winners, making them eligible for a $1 million bonus if either also won the US Open, a feat which they both accomplished, hence receiving the largest paychecks to date for a single tennis tournament, totalling $3.6 million each.
Document 9: 2012 US Open Series: In tennis, the ninth edition of the US Open Series (known as Emirates Airline US Open Series for sponsorships reasons), included twelve hard court tournaments that started on July 9, 2012 in Stanford and concluded in Winston-Salem for the men and in New Haven for the women on August 26, 2012. This edition scheduled four separate men's tournaments and four women's tournaments – the Citi Open and the Western & Southern Open hosted both a men's and women's event. The series was headlined by two ATP World Tour Masters 1000 and two WTA Premier 5 events. The series winners were Novak Djokovic and Petra Kvitová.
Document 10: 2014 Rafael Nadal tennis season: The 2014 Rafael Nadal tennis season officially began on 30 December 2013 with the start of the 2014 Qatar ExxonMobil Open. This season saw Nadal suffer from injuries that included a back injury, a wrist injury, and appendicitis. After losing in the final of the Australian Open where he suffered from a back injury, Nadal failed to defend his titles at Indian Wells, Barcelona, and Rome. He rebounded by claiming his 9th French Open. Prior to the North American hardcourt season, Nadal would experience another injury, this time a wrist injury which forced him to withdraw from the Rogers Cup, the Western & Southern Open and the US Open where he was the defending champion. Nadal announced on 24 October that he would not be competing for rest of the season due to appendicitis and eventually underwent surgery. Despite an injury plagued season, Nadal still ended the year at No. 3 with 4 titles.
Document 11: 2015 US Open Series: In tennis, the 2015 US Open Series (known as Emirates Airline US Open Series for sponsorship reasons) is the twelfth edition of the US Open Series, which includes a group of hard court tournaments that starts on July 27, 2015 in Atlanta and concluded in Winston-Salem for the men and in New Haven for the women on August 30, 2015. This edition consists of three separate men's tournaments and three women's tournaments, with the Western & Southern Open hosting both a men's and women's event. The series is headlined by two ATP World Tour Masters 1000 and two WTA Premier 5 events.
Document 12: Rafael Nadal: Rafael "Rafa" Nadal Parera (] , ] ; born 3 June 1986) is a Spanish professional tennis player, currently ranked World No. 1 in men's singles. Known as "The King of Clay", he is widely regarded as the greatest clay-court player in history. His evolution into an all-court threat has established him as one of the greatest tennis players ever.
Document 13: 2014 US Open Series: In tennis, the 2014 US Open Series (known as Emirates Airline US Open Series for sponsorship reasons) was the eleventh edition of the US Open Series, which included nine hard court tournaments that started on July 21, 2014 in Atlanta and concluded in Winston-Salem for the men and in New Haven for the women on August 23, 2014. This edition consisted of four separate men's tournaments and three women's tournaments, with the Western & Southern Open hosting both a men's and women's event. The series was headlined by two ATP World Tour Masters 1000 and two WTA Premier 5 events. Milos Raonic and Serena Williams were the US Open Series champions in 2014. Serena Williams improved the biggest payout in professional tennis history record which she previously shared with Rafael Nadal. The new record was then set at $4 million.
Document 14: 2017 US Open Series: In tennis, the 2017 US Open Series was the fourteenth edition of the US Open Series, which comprised a group of hard court tournaments that started on July 24, 2017 in Atlanta and concluded in Connecticut for the women and in Winston-Salem for the men on August 26, 2017. This edition consisted of three separate men's tournaments and three women's tournaments, with the Western & Southern Open hosting both a men's and women's event. The series was headlined by two ATP World Tour Masters 1000 and two WTA Premier 5 events.
Document 15: 2012 Barcelona Open Banc Sabadell: The 2012 Barcelona Open Banco Sabadell (also known as the "Torneo Godó") was a men's tennis tournament played on outdoor clay courts. It was the 60th edition of the event and it was part of the ATP World Tour 500 series of the 2012 ATP World Tour. It took place at the Real Club de Tenis Barcelona in Barcelona, Catalonia, Spain, from April 23 through April 29, 2010. Rafael Nadal won the singles title.
ground_truth: Rafael "Rafa" Nadal Parera