The 23-year-old Monfils was playing his first clay tournament of the season and was outclassed by a dominant Nadal, who has not lost a service game in his three matches in Madrid.

The second-seeded Spaniard pressured Monfils' serve from the start, and broke to go up 3-1 in the first set after the 12-seeded Frenchman double-faulted.

Another unforced error by Monfils gave Nadal a second break, before he comfortably served out the set.

Monfils earned his only two break points of the match when leading 2-1 in the second, but Nadal won the next four points and cruised through the rest of the set.

Nadal is undefeated on clay this season with a 13-0 record, having won titles in Monte Carlo and Rome and is looking for a record 18th Masters title. Nadal and Andre Agassi have won 17 each.

Nadal will play fellow Spaniard Nicolas Almagro, who beat Jurgen Melzer 6-3, 6-1 in the only quarterfinal not to feature a seeded player.

Almagro broke Melzer twice to close out the first set before racing to a 3-0 lead in the second with some solid serving.

The 24-year-old had another break before securing his first ATP Masters semifinal spot.

Fourth-seeded Venus Williams reached the semifinals, beating Samantha Stosur 6-3, 6-3.

In a strong performance Williams broke the eighth-seeded Stosur's serve four times.

Williams, who will climb to the second ranking behind sister Serena next week, has now won all four of her meetings with the Australian.

"Her game plan seemed to be to just attack everything," Williams said. "She played well. But there were some key points, and at the end, from 4-3, I felt like I went on autopilot, so it felt good." Despite the defeat, Stosur's recent form is projected to send her to seventh in the rankings next week, the highest held by an Australian since Wendy Turnbull in 1985.

Williams will play Shahar Peer of Israel, who beat Li Na of China 6-4, 3-6, 6-4.

Earlier, Lucie Safarova progressed to the semifinals with a 6-1, 1-6, 6-4 victory over 16th-seeded Nadia Petrova.

It was the Czech player's first win over Russia's Petrova in four attempts.

Safarova has had an impressive clay-court season, reaching the quarterfinals in Stuttgart and Rome and she will next face either seventh-seeded Jelena Jankovic or Aravane Rezai.