Question: What three time defending female champion has ranked number one in Women's Singles tennis?

full_context: Document 1: Venus Williams: Venus Ebony Starr Williams (born June 17, 1980) is an American professional tennis player who is currently ranked world No. 5 in the WTA singles rankings. She is generally regarded as one of the all-time greats of women's tennis and, along with younger sister Serena Williams, is credited with ushering in a new era of power and athleticism on the women's professional tennis tour.
Document 2: 1984 US Open – Women's Singles: Martina Navratilova was the defending champion, and she successfully defended her title, defeating last year's finalist Chris Evert in the final. This victory was Navratilova's sixth straight Major title, tying her with Margaret Court and Maureen Connolly for the most consecutive Grand Slam titles won. Navratilova became the first player to win Grand Slams on clay, grass and hard court on the same calendar year, after having won the French Open and Wimbledon.
Document 3: 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 4: Maria Sharapova: Maria Yuryevna Sharapova (Russian: Мари́я Ю́рьевна Шара́пова ; ] ; born April 19, 1987) is a Russian professional tennis player. A United States resident since 1994, Sharapova has competed on the WTA tour since 2001. She has been ranked world No. 1 in singles by the WTA on five separate occasions, for a total of 21 weeks. She is one of ten women, and the only Russian, to hold the career Grand Slam. She is also an Olympic medalist, having earned silver for Russia in women's singles at the 2012 Summer Olympics in London.
Document 5: Steffi Graf: Stefanie Maria "Steffi" Graf (] ; born 14 June 1969) is a German former tennis player, who was ranked world No. 1 during her career. Graf won 22 Grand Slam singles titles. Her 22 singles titles put her second on the list of Major wins in the female competition since the introduction of the Open Era in 1968 and is third all-time behind Margaret Court (24) and Serena Williams (23). In 1988, she became the first and only tennis player (male or female) to achieve the Golden Slam by winning all four Grand Slam singles titles and the Olympic gold medal in the same calendar year. Furthermore, she is the only tennis player to have won each Grand Slam event at least four times.
Document 6: 2009 Serena Williams tennis season: Serena Williams's 2009 tennis season officially began at the 2009 Medibank International Sydney. Williams finished the year ranked world no. 1 for the second time in her career, having played in 16 tournaments, more than any other year. She also broke the record previously set by Justine Henin for the most prize money earned by a female tennis player in one year, with Williams earning $6,545,586. In doubles, she finished the year ranked world no. 3, despite playing only six tournaments as a pair. She won five Grand Slam titles, putting her total Grand Slam titles at 23.
Document 7: 2017 French Open – Women's Singles: Garbiñe Muguruza was the defending champion, but lost to Kristina Mladenovic in the fourth round. Muguruza's defeat—along with those of Svetlana Kuznetsova, Venus Williams and Samantha Stosur in the same round—ensured that for the first time in a Grand Slam tournament since the 1979 Australian Open, none of the quarterfinalists were former Grand Slam champions.
Document 8: 2016 Wimbledon Championships – Women's Singles: Serena Williams was the defending champion and successfully defended her title, defeating Angelique Kerber in the final, 7–5, 6–3. By winning her seventh Wimbledon title, Williams equaled Steffi Graf's Open Era record of 22 major singles titles. This was also the first time two women contested two major finals against one another in a single season since Amélie Mauresmo and Justine Henin-Hardenne met in the 2006 Australian Open and Wimbledon finals.
Document 9: 2015 US Open – Women's Singles: Serena Williams was the three-time defending champion, but she lost in the semifinals to unseeded Roberta Vinci. In doing so, Williams missed the opportunity to become only the fourth woman (after Maureen Connolly, Margaret Court and Steffi Graf) to complete a calendar year Grand Slam by winning the tournament. She was also trying to set a new Open Era record by winning her seventh US Open singles title.
Document 10: 2017 US Open – Women's Singles: Angelique Kerber was the defending champion, but lost in the first round to Naomi Osaka. This loss would cause Kerber to fall outside the top ten in the WTA rankings for the first time since 2015.
Document 11: List of WTA number 1 ranked tennis players: The list of WTA number 1 ranked players shows the professional women's tennis players who have been or currently are ranked world No. 1 by the Women's Tennis Association, along with the dates of first reaching and losing that spot. Since the WTA began producing computerized rankings on November 3, 1975, 24 women have reached the highest singles ranking and 35 women the highest doubles ranking. The WTA Rankings are a merit-based method for determining the rankings in women's tennis and the WTA releases ranking points weekly based on how players fared during the previous 52 weeks.
Document 12: 2016 Australian Open – Women's Singles: Serena Williams was the defending champion, and attempting for the second time to equal Steffi Graf's career record of 22 major singles titles. She lost in the final to Angelique Kerber, 4–6, 6–3, 4–6. Kerber became the first woman to win a singles major title after having saved a match point in the first round, which she did against Misaki Doi. She is also the first German of either sex to win a major since Graf at the 1999 French Open.
Document 13: Serena Williams: Serena Jameka Williams (born September 26, 1981) is an American professional tennis player. The Women's Tennis Association (WTA) has ranked her world No. 1 in singles on eight occasions, from 2002 to 2017. She became the world No. 1 for the first time on July 8, 2002. On the sixth occasion, she held the ranking for 186 consecutive weeks, tying the record set by Steffi Graf for the most consecutive weeks as world No. 1 by a female tennis player. In total, she has been world No. 1 for 319 weeks, which ranks her third in the Open Era among female tennis players. Some commentators, players and sports writers regard her as the greatest female tennis player of all time.
Document 14: Williams sisters: The Williams sisters are two professional American tennis players: Venus Williams (b. 1980), a seven-time Grand Slam title winner (singles), and Serena Williams (b. 1981), twenty-three-time Grand Slam title winner (singles), both of whom were coached from an early age by their parents Richard Williams and Oracene Price. There is a noted professional rivalry between them – between the 2001 US Open and the 2017 Australian Open tournaments, they met in nine Grand Slam singles finals. They became the first two players, female or male, to play in 4 consecutive grand slam singles finals from the 2002 French Open to the 2003 Australian Open; Serena famously won all 4 to complete the first of two "Serena Slams". Between 2000 and 2016, a 17-year span, they collectively won 12 Wimbledon singles titles (Venus won 5 and Serena won 7). By winning the 2001 Australian Open women's doubles title, they became the 5th pair to complete the Career Doubles Grand Slam and the only pair to complete the Career Doubles Golden Slam. At the time, Venus and Serena were only 20 and 19 years old, respectively. Since then they have gone on to add another two Olympic gold medals in the 2008 Beijing Olympics and the 2012 London Olympics. Nearly a decade later, the duo would go on to win 4 consecutive grand slam doubles titles from 2009 Wimbledon through 2010 Roland Garros, which would catapult them to co-No. 1 doubles players on 7 June 2010. Two weeks later, on 21 June 2010, Serena would hold the No. 1 singles ranking and Venus would be right behind her at No. 2 in singles. Their most recent grand slam doubles titles came at the 2012 Wimbledon & 2016 Wimbledon events. They remain very close, often watching each other's matches in support, even after one of them has been knocked out of a tournament.
Document 15: Lindsay Davenport: Lindsay Ann Davenport Leach (born June 8, 1976) is an American former professional tennis player. She was ranked World No. 1 on eight different occasions, for a total of 98 weeks. Davenport is one of five women who have been the year-end World No. 1 at least four times (1998, 2001, 2004, and 2005) since 1975; the others are Chris Evert, Steffi Graf, Martina Navratilova and Serena Williams. She has achieved the No. 1 ranking in doubles as well.
ground_truth: Serena Williams