LONDON - Rafael Nadal made a successful transition to grass, defeating Juan Martin del Potro of Argentina 6-4, 6-4 Wednesday in the second round at Queen's Club.

Defending champion Lleyton Hewitt was upset by French qualifier Jo-Wilfried Tsonga 7-6 (5), 7-6 (2), and third-seeded Fernando Gonzalez edged Janko Tipsarevic 7-6 (7), 4-6, 7-6 (7).

Spain's Rafael Nadal plays a return to Argentine Juan Martin Del Potro during their tennis match at the Queen's Club grass court tennis championships in London Wednesday June 13, 2007. Nadal won the match 6-4, 6-4. [AP]

Fourth-seeded Novak Djokovic defeated American Robert Kendrick 3-6, 6-3, 6-2.

Nadal, the French Open champion, broke Del Potro in the ninth game of the first set, and then again to open the second.

"It's a big change from clay to grass, but I feel good," the top-seeded Nadal said. "The final last year in Wimbledon gave me a lot of confidence."

Nadal, who has won three straight French Open titles, lost to top-ranked Roger Federer at Wimbledon last year.

"(Del Potro) started serving very well but he stopped a little bit and I could start to return more balls," Nadal said. "But I was calm because with my serve I was winning the games ... and that's important - especially on grass."

Nadal said he felt relaxed at the Wimbledon warm-up.

"I am coming here with not much pressure ... try to play good tennis, try to adapt as fast as possible to grass," Nadal said.

Hewitt, a three-time winner from 2000-02, was unable to break down Tsonga's big serve.

Hewitt lost the first set, then broke to lead 2-0 in the second. But the 121st-ranked Tsonga broke back in the seventh game and again for 6-5 when Hewitt netted an easy backhand volley.

The sixth-seeded Hewitt broke to take the match to a tiebreaker, where Tsonga conceded just two points.

Against Djokovic, Kendrick slipped and hurt his left knee in the second set.

"I was very careful how I move because usually on clay you slide, and here you cannot slide," Djokovic said. "You could see that Robert got a little injury. It could have been more serious. He was lucky."

Djokovic, a semifinalist at the French Open, made several errors early in the match, but he earned a break in the second set after Kendrick's three double-faults. Kendrick slid and twisted his knee in the next game.

Also, seventh-seeded Dmitry Tursunov of Russia defeated Frank Dancevic of Canada, 7-5, 6-1.

Arnaud Clement of France beat Chris Guccione of Australia 6-4, 6-4; Marin Cilic of Croatia defeated Alejandro Falla of Colombia 6-3, 6-4; and Ivo Karlovic of Croatia topped Alex Kuznetsov of the United States 2-6, 6-2, 7-6 (2).

Marat Safin lost the first set to two-time finalist Sebastien Grosjean of France. The Russian recovered to win the second set before rain ended play for the day at 6-3, 4-6, 4-4.