Question: Which of the following was the greatest service returner in the history of the game of tennis: Andre Agassi or Martina Hingis?

Context articles: Martina Hingis
Fernando Verdasco
1999 Porsche Tennis Grand Prix – Doubles
Andre Agassi
100 Greatest of All Time
Andre Agassi Tennis
Gil Reyes (tennis)
1999 Wimbledon Championships – Men's Singles
Agassi–Sampras rivalry
Agassi Tennis Generation

Martina Hingis (born 30 September 1980) is a Swiss professional tennis player, a former world No. 1 singles player and currently ranked world No. 2 in doubles by the WTA.
 She has spent a total of 209 weeks as the singles No. 1 and has won five Grand Slam singles titles, thirteen Grand Slam women's doubles titles, winning a calendar-year doubles Grand Slam in 1998, and seven Grand Slam mixed doubles titles; for a combined total of twenty-five major titles.
 In addition, she has won the season-ending WTA Championships two times in singles and three times in doubles, and an Olympic silver medal.
Fernando Verdasco Carmona (] ; (born 15 November 1983) is a Spanish professional tennis player.
 His career-high singles ranking is world No. 7, achieved in April 2009.
 Verdasco started playing tennis at four years of age and had a full-time coach when he was eight.
 As of 2009, Verdasco has been working in Las Vegas with Andre Agassi and his team, including Darren Cahill (Agassi's former coach) and Gil Reyes (Agassi's fitness coach).
The 1999 Porsche Tennis Grand Prix – Doubles was the tennis doubles event at the 1999 Porsche Tennis Grand Prix, the twenty-second edition of the most prestigious tournament in Baden-Württemberg.
 Lindsay Davenport and Natasha Zvereva were the defending champions, but they did not compete this year.
 Davenport competed with Martina Hingis as the first seed, while Zvereva competed with Elena Tatarkova.
 Davenport and Hingis withdrew in the quarterfinals due to Davenport having a wrist injury, whilst Tatarkova and Zvereva were knocked out in the first round.
Andre Kirk Agassi ( ; born April 29, 1970) is an American retired professional tennis player and former World No. 1 who was one of the sport's most dominant players from the early 1990s to the mid-2000s.
 Generally considered by critics and fellow players to be one of the greatest tennis players of all time, Agassi has been called the greatest service returner in the history of the game.
 Described by the BBC upon his retirement as "perhaps the biggest worldwide star in the sport's history", Agassi compiled performances that, along with his unorthodox apparel and attitude, saw him cited as one of the most charismatic players in history.
 As a result, he is credited for helping to revive the popularity of tennis during the 1990s.
100 Greatest of All Time was a television series of five one-hour episodes, produced and first aired by Tennis Channel in March 2012.
 It presented a list of the "100 greatest tennis players of all time", both men and women.
 The series was hosted by Jack Nicklaus, Jerry Rice, Wayne Gretzky, Lisa Leslie and Carl Lewis.
 Many retired tennis luminaries provided commentary, including Rod Laver, Billie Jean King, Chris Evert, Björn Borg, John McEnroe, Martina Navratilova, Pete Sampras, and Andre Agassi.
Andre Agassi Tennis is a tennis video game released in 1992, starring tennis legend Andre Agassi.
 The game was released for the Super NES, Genesis, Master System and Game Gear.
 It enjoyed a much belated release for mobile phones.
Gil Reyes (born 1952) is an American fitness trainer who spent seventeen years as the strength and conditioning trainer for retired tennis player Andre Agassi, from 1989 until Agassi's retirement in 2006.
 He was also considered one of Agassi's closest friends and confidants.
Pete Sampras was the two-time defending champion and won in the final 6–3, 6–4, 7–5 against Andre Agassi.
 With this victory, Sampras equalled Roy Emerson's record of twelve Grand Slam titles.
 Sampras' victory over Agassi in the final is often cited as one of his greatest performances.
 However, despite his victory, he lost his No. 1 ranking to Agassi the following day, when the ATP Tour rankings were updated.
Pete Sampras and Andre Agassi are retired professional men's tennis players who were both ranked World No. 1 during the 1990s, Sampras holding the world's top-rank spot for a then-record 286 weeks while Agassi held it for 101 weeks.
 With contrasting styles and temperaments, they played each other 34 times from 1989 through 2002, with Sampras winning 20 matches.
 It has been named as one of the greatest tennis rivalries of all time.
Agassi Tennis Generation is a tennis sports game, developed by Aqua Pacific and published by DreamCatcher Interactive for the PC, PlayStation 2 and Game Boy Advance.
 It was originally released for PC on August 12, 2002.
 The game features former tennis player Andre Agassi.