Question: Rafael Nadal defeated which 19-time Grand Slam titlist in the 2008 French Open for his fourth straight clay major title?
Context: 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. 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. Roger Federer won two Majors in 2009, the French Open, defeating Robin Söderling in the final, and the Wimbledon Championships with a victory over Andy Roddick.  In addition, Federer made two other Grand Slam finals, Australian Open losing to Rafael Nadal, and the US Open, losing to Juan Martín del Potro.  Federer went on to win two other Master Series 1000 tournaments: in Madrid over Rafael Nadal, and in Cincinnati over Novak Djokovic.  He lost in one 500 level event final in Basel to Djokovic.  During the year, Federer completed the Career Grand Slam by winning his first French Open title, and won a record fifteenth Grand Slam singles title, one more than Pete Sampras' mark of fourteen. The 2005 French Open was the 104th edition of the tournament.  Rafael Nadal, seeded fourth at his first French Open (but was actually ranked World No. 5 at the time after then-World No. 2 Lleyton Hewitt withdrew from the tournament due to injury), was a strong favorite to win the men's title after winning the Monte Carlo and Rome Masters, with Guillermo Coria, a 2004 finalist and 2005 runner-up to Nadal in both Monaco and Rome, calling Nadal the best clay-court player in the world prior to the tournament.  After defeating top seed Roger Federer in the semi-finals, Nadal defeated Argentina's Mariano Puerta to claim his first French Open title, and the first of four won consecutively from 2005 until 2008.  Nadal would go on to win the tournament a record ten times. 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. The Djokovic–Nadal rivalry is between tennis players Novak Djokovic and Rafael Nadal, who are engaged in a modern-day tennis rivalry.  The rivalry is the most prolific of the Open Era.  The two have faced each other 50 times with Djokovic leading 26–24.  Between 2006–2010, this rivalry was often overshadowed by the historic Federer–Nadal rivalry, but it started to become widely recognized after they played their first Grand Slam final against each other at the 2010 US Open.  The first half of the rivalry was dominated by Nadal (who led 16–7 after the 2010 season) while the second half has been dominated by Djokovic, winning 11 out the last 13 matches.  It is widely considered to be one of the greatest rivalries in tennis history.  It was the first rivalry to involve meetings at all 4 Grand Slam finals (including four consecutive Grand Slam finals) and a record 26 Masters Series matches.  Their 2012 Australian Open final is considered to be one of the greatest matches of all time, and Mary Carillo and John McEnroe both commented that that Australian Open match and the 2013 French Open semi-final match were the two best matches played on hard and clay courts respectively.  The Djokovic–Nadal rivalry was listed as the third greatest rivalry in the last decade in 2009 by "ATPworldtour.com" despite its only starting in 2006. 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. The 2012 French Open Men's Singles final was the championship tennis match of the Men's Singles tournament at the 2012 French Open.  In the final, Rafael Nadal defeated Novak Djokovic 6–4, 6–3, 2–6, 7–5 to win the match.  It was a Major final match of historic proportions, as Rafael Nadal was looking to become the first man to win seven French Open titles, thus breaking the record previously held by Björn Borg, who won six titles, and equalling the record held by Chris Evert, who won seven titles, whilst Novak Djokovic was looking to become the first man since Rod Laver in 1969 to hold each of the four Major titles simultaneously.  As well, Djokovic had defeated Nadal in the previous 3 Grand Slam finals and Nadal was trying to avoid coming runner-up in all 4 tournaments to the same player. 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. Rafael Nadal was the 3-time defending champion and retained his title, defeating Roger Federer in the final 6–1, 6–3, 6–0.  This was Federer's worst loss in a Grand Slam event in his entire
Answer: Roger Federer