In this task, you are given a set of context paragraphs, some supporting facts and an answer of a question. Your task is to generate question for given answer based on set of context paragraphs, supporting facts and an answer.

Context_1 : Unmatched is a documentary about tennis players Martina Navratilova and Chris Evert, their decades-long on-court rivalry and lifelong friendship, created for ESPN's "30 for 30" documentary series. Evert and Navratilova met in 80 matches, 60 finals and 14 grand slam finals with Navratilova sporting a 43-37 advantage. Context_2 : Roy Stanley Emerson (born 3 November 1936) is an Australian former World number one tennis player who won 12 Major singles titles and 16 Grand Slam tournament men's doubles titles. He is the only male player to have completed a Career Grand Slam (winning titles at all four Grand Slam events) in both singles and doubles. His 28 major titles are an all-time record for a male amateur player. Emerson is the first male player to win each amateur major title at least twice in his career. He is one of only eight men to win all four majors in his career. He was the first male player to win 12 majors. Along with Novak Djokovic, he is one of only two male players to win six Australian Championships. He won five of them consecutively (196367). His 12 wins have since been surpassed. Emerson is one of only five tennis players all-time to win multiple slam sets in two disciplines, only matched by Margaret Court, Martina Navratilova, Frank Sedgman and Serena Williams. Context_3 : Gisela Dulko (] ; born 30 January 1985, in Tigre) is a retired professional tennis player from Argentina. Although she enjoyed modest success in singles, reaching world no. 26 and winning four WTA Tour titles, her speciality was doubles, where she achieved a world no. 1 ranking. Partnering with Flavia Pennetta, Dulko won the 2010 WTA Tour Championships and the 2011 Australian Open in doubles. Dulko also reached the mixed doubles final at the 2011 US Open, with Edward Schwank. She is remembered for her ability to upset the big names on the tour, such as Maria Sharapova in the second round of Wimbledon in 2009; and Samantha Stosur in the third round of Roland Garros in 2011. She also beat Martina Navratilova in the second round of Wimbledon in 2004 in Navratilova's final Grand Slam singles match. Context_4 : Jack Sock (born September 24, 1992) is an American professional tennis player who is currently ranked world No. 21 in men's singles by the Association of Tennis Professionals (ATP). He is one of the top ranked American men in both singles and doubles on the ATP Tour. A former junior US Open champion, Sock's singles success is highlighted by 7 ATP finals, including three titles. His career best result came at the 2017 Indian Wells Masters, where he reached the semi-finals. Context_5 : Judy Hill Nelson is an American author best known for her 19831991 romance (while married) with and eventual palimony suit against women's tennis star Martina Navratilova. After their break-up, Nelson was the author of the books "Love Match: Nelson vs. Navratilova", chronicling her relationship with the tennis star, and "Choices: My Journey After Leaving My Husband for Martina and a Lesbian Life", chronicling her coming out as a lesbian in the mid-1980s. She is the mother of two sons. Context_6 : Meredith McGrath (born April 28, 1971) is a former professional tennis player. She was born in Midland, Michigan, USA and made her debut on the Women's Tennis Association tour in 1988. In her eight-year professional career Meredith achieved career-high world rankings of #18 in singles and #4 in doubles. She notched victories over such players as Martina Navratilova, Martina Hingis, Anna Kournikova, Jana Novotna, Arantxa Sanchez Vicario, and Gigi Fernandez. The pinnacle of her career came in reaching the singles semi-finals and doubles finals at the 1996 Wimbledon Championships. In the singles she defeated Mana Endo, Amanda Coetzer, Nancy Feber, Katarina Studenikova and Mary Joe Fernandez before losing to Arantxa Sanchez Vicario. An injury the week prior to the 1996 Wimbledon Championships eventually ended her career. Meredith won the 1995 U.S. Open Mixed Doubles Championship (she was runner-up in 1989) and was the runner-up in the 1994 Australian Open Doubles Championship. Meredith won three WTA singles titles: Oklahoma City and Eastbourne in 1994 and Birmingham in 1996, 25 doubles titles. She retired with a 14083 record in singles and a 18940 record in doubles. Meredith was recognized by Tennis Magazine as the WTA Comeback Player of the Year as she overcame near career-ending injuries to having her most successful competitive season in 1996. In 1994 she received the WTA Tour Comeback Player of the Year. Context_7 : Francis "Frank" Arthur Sedgman, AM (born 29 October 1927) is a retired former World No. 1 amateur tennis champion. In his 1979 autobiography Jack Kramer, the long-time tennis promoter and great player himself, included Sedgman in his list of the 21 greatest players of all time. Sedgman, Kramer wrote, "was as quick as anybody who ever played the game, but he couldn't keep the heat on." Sedgman is one of only five tennis players all-time to win a multiple slam set in two disciplines, matching Margaret Court, Roy Emerson, Martina Navratilova and Serena Williams. In 1951 he and Ken McGregor won the men's doubles Grand Slam. Sedgman turned professional in 1953. Context_8 : Maria Fernanda Barbato Alves (born 17 April 1983), also known as Nanda Alves, is a Brazilian tennis player. She is currently coached by her father Carlos and former tennis player Thomaz Koch. As of 1 March 2010, Alves is ranked World No. 262, and is the highest ranked Brazilian player. She has enjoyed success at the ITF Circuit, winning 18 singles and 26 doubles titles so far. Alves made her WTA Tour debut at Copa Colsanitas in 2004, and has played qualifications for all four Grand Slam events. Partnering with Vanessa Henke, Alves took part in the 2005 Australian Open doubles event, but they lost to Daniela Hantuchova and Martina Navratilova in the first round. Context_9 : Martina Navratilova (Czech: "Martina Navratilova" ] ; born Martina Subertova ] ; October 18, 1956) is a former Czechoslovak and later American tennis player and coach. In 2005, "Tennis" magazine selected her as the greatest female tennis player for the years 1965 through 2005 and she is considered one of the best, if not the best, female tennis players of all time. Context_10 : Bettina Fulco (born 23 October 1968) is a retired Argentine professional women's tennis player. She reached her highest ranking of No. 23 on October 10, 1988. Fulco began playing tennis at age 10 at the University Club in her hometown of Mar Del Plata, having been inspired to start because of the increased interest in the sport in Argentina due to Guillermo Vilas' success. As a junior, Bettina was among the best in the world, reaching the finals of the Orange Bowl 18-and-under championships in 1986, and finishing second in the rankings in 1986. She turned professional in 1987. Like many South American players, Bettina Fulco was considered a clay court specialist, and reached the quarterfinals of the French Open in 1988. Bettina beat Martina Navratilova in Houston 1994 for her biggest career victory. She also achieved victories over Conchita Martinez, Arantxa Sanchez Vicario, Hana Mandlikova, Katerina Maleeva, Manuela Maleeva, Magdalena Maleeva, Claudia Kohde-Kilsch, Lori McNeil and Nathalie Tauziat. She retired from professional tennis in 1998. fact_1 : Martina Navratilova (Czech: "Martina Navratilova" ] ; born Martina Subertova ] ; October 18, 1956) is a former Czechoslovak and later American tennis player and coach. fact_2 : In 2005, "Tennis" magazine selected her as the greatest female tennis player for the years 1965 through 2005 and she is considered one of the best, if not the best, female tennis players of all time. fact_3 : Jack Sock (born September 24, 1992) is an American professional tennis player who is currently ranked world No. 21 in men's singles by the Association of Tennis Professionals (ATP). Answer: Martina Navratilova
Who is a considered a better tennis player, Martina Navratilova or Jack Sock?