WIMBLEDON, England - Trying to switch directions while chasing a shot beneath the Royal Box at Wimbledon's Centre Court, Rafael Nadal lost his footing and took a nasty-looking spill, flinging his racket in the air while he fell.

The Spaniard was OK, even if he did land on his back behind the baseline and glanced overhead to make sure his equipment wasn't going to nail him in the noggin. Nadal toweled off and, four games later, wrapped up a 6-2, 6-4, 6-4 victory over Japan's Kei Nishikori in the first round at the All England Club.

It was merely a coincidence, of course, that Nadal's third-set tumble Tuesday left him in nearly the same position, and in the same spot, that he found himself the last time he played a match at Wimbledon, almost two full years ago. Except on that occasion, he was dropping to the grass on purpose to celebrate beating Roger Federer in the 2008 final.

Nadal didn't defend that championship, withdrawing last year because of painful tendinitis in his knees. Now he's healthy, reinstated at No. 1 in the rankings, once again the reigning French Open champion - and back on what he called "probably the most beautiful and emblematic court in the world."

Said Nadal: "I'm happy to be here again."

He's also happy to be able to scamper around a court as only he can, knowing that his knees won't let him down.

Another past Wimbledon winner coming off an injury, Maria Sharapova, also played Tuesday, and also won convincingly, eliminating 127th-ranked Anastasia Pivovarova of Russia 6-1, 6-0 in only 54 minutes.

Sharapova had right shoulder surgery in October 2008 and was sidelined for about 10 months, then was hampered by a right elbow injury this season. She's had to toy with her service motion, in addition to confronting worries about when she would regain the strokes that helped her win Wimbledon at age 17 in 2004, then two other Grand Slam titles.

"You never know until things happen. I mean, I believed that I would be back. Did I really know? I assumed, and I had a lot of belief in myself, and I had a tremendous amount of support from the people around me," said Sharapova, who won 30 of 37 points on her serve Tuesday. "But you never really know till it happens."

And these days, is her best tennis enough to beat anyone?

"Absolutely," Sharapova said.

Joining her in the second round were defending champion Serena Williams, who pounded 15 aces in a 6-0, 6-4 victory over 17-year-old Michelle Larcher de Brito of Portugal to improve to 43-0 in first-round Grand Slam matches; two-time major champion Svetlana Kuznetsova, who overcame 16 aces by Akgul Amanmuradova of Uzbekistan and won 6-2, 6-7 (5), 6-4; and No. 3 Caroline Wozniacki, No. 7 Agnieszka Radwanska, No. 9 Li Na and No. 10 Flavia Pennetta.

But No. 6 Samantha Stosur, the runner-up at the French Open a little more than two weeks ago, lost to 80th-ranked qualifier Kaia Kanepi of Estonia 6-4, 6-4. The woman who beat Stosur at Roland Garros, first-time major champion Francesca Schiavone, bowed out of Wimbledon on Monday.

The only other seeded woman who lost Tuesday was No. 25 Lucie Safarova of the Czech Republic.

Four seeded men exited, and three are from Spain: No. 8 Fernando Verdasco, No. 19 Nicolas Almagro, and No. 14 Juan Carlos Ferrero, the 2003 French Open champion. The other was No. 24 Marcos Baghdatis of Cyprus, the 2006 Australian Open runner-up.

Winners included No. 4 Andy Murray, two-time French Open finalist Robin Soderling, No. 9 David Ferrer, No. 10 Jo-Wilfried Tsonga, and No. 18 Sam Querrey of Santa Monica, Calif. Murray is the only British man or woman still left, making this the first time in tournament history that the hosts have but one representative in the second round.

"It's not great, is it?" Murray said.

For all of Nadal's success at the French Open - he is 38-1 there, with five championships in six years - he considers his Wimbledon trophy his most significant accomplishment. Having to pull out of the grass-court Grand Slam tournament in 2009 was part of a tough season that also included his only loss at Roland Garros and, away from the court, his parents' separation.

"We had many problems last year, with the knees and many things. After Roland Garros, it was a difficult decision not playing here," said Toni Nadal, Rafael's uncle and coach. "When you are the (defending) champion, you want to at least come here to play. But that's life."

Nadal is seeded second behind six-time Wimbledon champion Federer, who struggled in his first match Monday, coming back to win after dropping the opening two sets. Nadal faced no such dangers against the 189th-ranked Nishikori, who was honored as the 2008 ATP newcomer of the year after becoming the first Japanese man since 1937 to reach the U.S. Open's fourth round, but then missed time with a right elbow injury.

"If I hit one easy shot, he's going to hit winners. So it wasn't easy for me," Nishikori said. "I think he was playing really well today."

If Nishikori is still trying to re-establish himself, Nadal clearly has recovered from his knee issues, going 22-0 on clay this spring.

Now his indefatigable style is once again on display at Wimbledon.

"This year, I worked a lot to be here," Nadal said. "I'm perfect. Concentrated. Very motivated."