Nadal scored 21 unanswered points in the second set to beat Kristof Vliegen of Belgium 6-1, 6-1. The Spaniard last lost on clay more than two years ago.

"I'm defending good. I'm running good," Nadal said. "I'm feeling better on clay in my movements."

Federer, who has reached the quarterfinals at Monte Carlo three straight times, defeated Lee Hyung-taik 6-4, 6-3. Federer next plays 12th-seeded David Ferrer of Spain, who has never won a set against Federer in five losses.

Nadal will next face Philipp Kohlschreiber of Germany, who beat Guillermo Garcia Lopez of Spain 5-7, 7-6 (4), 7-6 (4).

Nadal dropped serve at the start of the second set but won the next 21 points before Vliegen got to 15-15 at 5-1 down. Vliegen managed to win another point when Nadal hit an unforced error on his first match point.

"I'm happy with my level," Nadal said. "I played a very, very comfortable match. Very good forehands and backhands."

Federer, a 10-time Grand Slam champion, lost last year to Nadal in the finals at the Monte Carlo Masters and the French Open — the only major Federer has not won.

Ferrer recovered from 4-1 down in the first set to beat No. 6 Novak Djokovic of Serbia 7-5, 6-4.

"He's an excellent player, especially on clay," Federer said of Ferrer. "This is where he's got the most confidence, so it's going to be difficult to beat him here. Having not lost a set doesn't mean much."

Federer set up match point with a serve-and-volley winner and then earned the victory when Lee hit a powerful backhand long.

Lee broke the top-ranked Swiss star in the third game of the first set, but Federer broke back in the fourth and took control.

"Maybe it was just my timing in the beginning," Federer said. "I was just shanking a lot of the balls and not able to keep the ball in play. Once I was able to do that, I was in total control. So I never felt worried too much today."

Federer's match against Lee was played under cloudy skies, and he beat Andreas Seppi on Tuesday in bright sunshine.

"I had a really quick court (for) the first match and a really slow court the second match," Federer said. "It's really good (that) I played in both conditions so I'm ready for anything."

Also Thursday, 10th-seeded Tomas Berdych rallied to beat No. 5 Tommy Robredo 1-6, 6-3, 6-2; No. 11 Richard Gasquet edged seventh-seeded Ivan Ljubicic of Croatia 6-3, 6-7 (5), 7-5; and No. 16 Juan Carlos Ferrero downed Igor Andreev of Russia 6-3, 6-3.

Berdych, whose previous best at Monte Carlo was reaching the second round, will face Robin Soderling in Friday's quarterfinals. The Swede beat Max Mirnyi of Belarus 7-6 (5), 6-2.

Gasquet's aggressive play earned him five breaks against Ljubicic. The Frenchman next plays Ferrero, a two-time Monte Carlo champion.