Question: Which of the following won 19 Grand Slam single titles as a professional tennis player: Roger Federer or Jonathan Stark?
Context: Roger Federer reached all four Grand Slam finals in 2006, winning three of them.  His only Grand Slam loss came against Rafael Nadal in the French Open final in four sets, 6–1, 1–6, 4–6, 6–7(4).  This was the first time they had met in a Major final.  In the other three Grand Slams of 2006, Federer defeated Nadal in the final of the Wimbledon Championships, 6–0, 7–6(5), 6–7(2), 6–3.  He defeated Marcos Baghdatis, 5–7, 7–5, 6–0, 6–2, at the Australian Open and Andy Roddick, 6–2, 4–6, 7–5, 6–1, at the US Open.  In addition, Federer made it to six ATP Masters Series 1000 finals, winning four on hard surfaces and losing two on clay to Nadal.  Also, Federer won one ATP 500 series event in Tokyo, three ATP 250 series events in Doha, Halle, and Basel, and captured the Year-End Championships for the third time in his career.  In December 2011, Stephen Tignor, chief editorial writer for Tennis.com, ranked Federer's 2006 season as the second greatest season of all-time during the open era; behind only Rod Laver's Calendar Grand Slam year of 1969. The 2017 Australian Open Men's Singles final was the championship tennis match of the Men's Singles tournament at the 2017 Australian Open.  It was contested between Roger Federer and Rafael Nadal, ranked 17th and 9th in the world respectively.  It was their record ninth meeting in a Grand Slam final in their rivalry, and their 1st meeting in a Grand Slam final since the 2011 French Open.  In a rematch of the 2009 Australian Open final, which Nadal won in 5 sets, Roger Federer won the duel in 5 sets, beating Nadal for the first time in a Grand Slam since the 2007 Wimbledon final.  He also trailed Nadal 3–1 in the final set but won 5 games in a row to win the title.  This ended a 6-match losing streak against Nadal in Grand Slam events.  Having lost all of their previous three encounters, this was the first time Federer defeated Nadal at the Australian Open and also marked Federer's first Grand Slam victory over Nadal outside the grass courts of Wimbledon.  Federer extended his record of Grand Slam men's singles titles to 18, marking the third time he broke his own all-time record, after breaking the previous record of 14, held by Pete Sampras. Nicolas Pierre Armand Mahut (] ; born 21 January 1982) is a French professional tennis player.  In singles, he reached a career-high Association of Tennis Professionals (ATP) ranking of world No. 37 on 5 May 2014.  In doubles, he reached a career-high ATP ranking of world No. 1 on 6 June 2016.  Mahut is well known for being skilled on grass, on which he has won the third-most number of titles amongst active players in singles behind Roger Federer (15) and Andy Murray (8) and tying with Rafael Nadal (4); he also has the most singles titles on grass won over the age of 30 amongst active players, tying with Federer (4).  He is a distinguished doubles player, having been ranked world No. 1, and has reached all four Grand Slam finals in men's doubles, including winning the 2015 US Open and 2016 Wimbledon men's doubles titles. Roger Federer (] ; born 8 August 1981) is a Swiss professional tennis player who is currently ranked world No. 2 in men's singles tennis by the Association of Tennis Professionals (ATP).  Federer has won 19 Grand Slam singles titles, the most in history for a male tennis player and has held the world No. 1 spot in the ATP rankings for a record total of 302 weeks, including a record 237 consecutive weeks.  After turning professional in 1998, he was continuously ranked in the top ten from October 2002 to November 2016. Ai Sugiyama (杉山愛 , Sugiyama Ai , born July 5, 1975) is a Japanese former professional tennis player.  She reached No. 1 in women's doubles on the WTA Tour.  Her career-high singles ranking was No. 8, achieved on February 9, 2004.  In her career, she won 6 singles titles and 37 doubles titles, including three Grand Slam women's doubles titles (one with Frenchwoman Julie Halard-Decugis and two partnering Belgian Kim Clijsters).  Sugiyama held the all-time record, for both male and female players, for her 62 consecutive Grand Slam main draw appearances, until she was surpassed by Roger Federer at the 2015 Wimbledon Championships. This is a list of the main career statistics of Swiss professional tennis player Roger Federer.  All statistics are according to the ATP World Tour website.  To date, Federer has won ninety-three ATP singles titles including a record nineteen Grand Slam singles titles, twenty-six ATP Masters 1000 titles and a record six Year-End Championships.  Federer was also a Gold Medalist in men's doubles with Stan Wawrinka at the 2008 Beijing Olympics and a Silver Medalist in men's singles at the 2012 London Olympics.  Representing Switzerland, Federer also won the 2001 Hopman Cup and the 2014 Davis Cup. Stefan Bengt Edberg (] ; born 19 January 1966) is a Swedish former world no. 1 professional tennis player (in both singles and doubles).  A major proponent of the serve-and-volley style of tennis, he won six Grand Slam singles titles and three Grand Slam men's doubles titles between 1985 and 1996.  He also won the Masters Grand Prix and was a part of the Swedish Davis Cup-winning-team four times.  In addition he won four Masters Series titles, four Championship Series titles and the unofficial Olympic tournament 1984, was ranked in the singles top 10 for ten successive years, 9 years in the top 5, and is considered one of the greatest players of his era.  Edberg began coaching Roger Federer in January 2014, with this partnership ending in December 2015. The 2009 Australian Open Men's Singles final was the championship tennis match of the Men's Singles tournament at the 2009 Australian Open.  It was contested between the world's top two players for much of the previous four years, Rafael Nadal and Roger Federer, then ranked first and second in the world respectively.  It was their seventh (out of nine) meeting in a Grand Slam final, but their first outside of either the French Open or Wimbledon, followed by their most recent meeting at the final of the 2017 Australian Open.  This was Rafael Nadal's first Grand Slam hard court final while it was Roger Federer's ninth and at the time he was yet to lose in a Grand Slam hard court final. Jonathan Stark (born April 3, 1971) is a former professional tennis player from the United States.  During his career he won two Grand Slam doubles titles (the 1994 French Open Men's Doubles and the 1995 Wimbledon Championships Mixed Doubles).  Stark reached the World No. 1 doubles ranking in 1994. Roger Federer won one slam in 2008, which came at the 2008 US Open, defeating Briton Andy Murray, 6–2, 7–5, 6–2.  Federer was defeated by Rafael Nadal in two Grand Slam finals: at the 2008 French Open, which he lost 1–6, 3–6, 0–6, and at the 2008 Wimbledon Championships in a five-setter, 4–6, 4–6, 7–6, 7–6, 7–9, when he was going for six straight wins to break Björn Borg's record.  At the 2008 Australian Open, Federer lost in the semifinals to Novak Djokovic, ending his record of 10 consecutive Major finals.  Roger Federer lost twice in Master Series 1000 Finals on clay to Nadal at Monte Carlo and Hamburg.  However, Federer was able to capture three more victories in 250-level events at Estoril, Halle, and Basel.
Answer: Roger Federer