Rafael Nadal defeated which 19-time Grand Slam titlist in the 2008 French Open for his fourth straight clay major title?
2008 French Open – Men's Singles: Rafael Nadal was the 3-time defending champion and retained his title, defeating Roger Federer in the final 6–1, 6–3, 6–0. Roger Federer:  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.
Roger Federer