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

Context paragraphs:
Qu Yun: Qu Yun (; born June 5, 1978 in Hangzhou, Zhejiang) is a former butterfly stroke swimmer from China.  In 1993, she broke the Asian Record in women's 200m buttfly at 2:08.53 in Chinese National Games which ranked number 1 in the world in 1993.  She also won silver in 100m butterfly at 59.27 which ranked number 2 in the world.  She competed in 1994 World Aquatics Championships and won Silver medals in women's 100 m (59.69) and 200 m butterfly (2:07.40).  She finished behind her compatriot, Liu Limin in both race.  The time in 200m butterfly was her career best.  In October, 1994, she set career best in women's 100m butterfly (58.70) in Asian Games to win silver medal behind Liu Limin.  In 1996, she won the Olympic trial in April with a time of 2:09.66 in 200m butterfly over Liu Limin and she competed in women's 200 m butterfly at the 1996 Summer Olympics and placed 4th.  In 1997, Qu made a come back and scored double win (100 butterfly in 59.36 and 200m butterfly in 2:08.10) in Chinese national Games.  Her time in 200m butterfly ranked number 2 in the world behind Susan O'Neill.  In January, 1998, She competed women's 200m butterfly in 1998 World Aquatics Championships and finished 5th (2:10.49).  This was her last appearance in major competition.  She retired after 2001 Chinese National Games.

Martin Legner: Martin Legner (born December 17, 1961 in Tyrol) is a professional Austrian wheelchair tennis player who has been ranked number one for wheelchair doubles.  Legner has won the Australian Open doubles title four times with Robin Ammerlaan since 2000, and has won eight doubles titles.  Six of them are Australian Open Titles, the other two being French Open titles.  He has won one French Open and one Australian Open singles title.  He has represented his country at every Summer Paralympics since 1992, and has competed in both singles and doubles at all of those Games.  His favorite surfaces are clay and hard court.  As of April 7, 2007 his highest singles rank was number three.  Also of that date, he was ranked number six for singles and doubles.  His wheelchair is manufactured by Kronbickler.  Legner is a fan of Bob Marley, and listens to his music aside from tennis.

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.

1987 US Open – Men's Singles: Ivan Lendl was the two time defending champion and retained his title again, defeating Mats Wilander 6–7, 6–0, 7–6, 6–4 in the final to win the Men's Singles tennis title at the 1987 US Open.  It was also the 6th consecutive US Open final of Lendl's record 8 consecutive US Open finals.

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.

Zoja Rudnova: Zoja Rudnova (19 August 1946 - 12 March 2014) is a former female table tennis player from Soviet Union.  From 1964 to 1976 she won several medals in singles, doubles, and team events in the Table Tennis European Championships and in the World Table Tennis Championships.  She was twice European champion in women singles, three time European champion with the USSR team, once in women doubles and four times in mixed doubles.  She was the first woman ever to become an absolute European champion in 1970 winning all four possible gold medals (singles, team, doubles, mixed doubles) - great feat which has only been repeated once since.

Netball in Europe: Netball is primarily played in Commonwealth countries, which were heavily involved in standardising the rules for netball.  The Federation of European Netball Associations (FENA), sometimes referred to as Netball Europe, is the governing body for netball in the Europe netball region.  The organisation was created in 1989/1990.  As of August 2016, the English women's national team was ranked number three in the world, Wales is ranked number eight, Scotland is ranked number ten, Northern Ireland is ranked number twelve, Ireland are ranked number twenty-five, Gibraltar are ranked number thirty-one, Malta are ranked number thirty-two and Switzerland are ranked thirty-four.

Mercedes Martinez: Mercedes Martinez (born November 17, 1980) is an American professional wrestler, currently performing for Shimmer Women Athletes, as well as numerous independent promotions including World Xtreme Wrestling, Full Impact Pro, and Women Superstars Uncensored.  She is the current and three-time WSU Champion, the current two-time Shimmer Champion. , WXW Women's champion, nCw Female Champion, and one half of the current Shine Tag Team Champions.

1988 US Open – Men's Singles: Mats Wilander defeated three time defending champion Ivan Lendl, 6–4, 4–6, 6–3, 5–7, 6–4 in the final to win the Men's Singles title at the 1988 US Open.  This was the longest US Open men's final in history, clocking in at four hours and fifty-four minutes; this record was matched in 2012 when Andy Murray defeated Novak Djokovic in that year's final.

1981 US Open – Men's Singles: John McEnroe was the two time defending champion and retained his title again, defeating Björn Borg 4–6, 6–2, 6–4, 6–3 in the final to win the Men's Singles tennis title at the 1981 US Open in a repeat of the previous years final, and which turned out to be Borg's final grand slam match.

Supporting paragraphs: 2015 US Open – Women's Singles Serena Williams
Answer: Serena Williams