WIMBLEDON, England - About 18 months removed from his last Grand Slam championship, Roger Federer figures it's about time for another - his record-tying seventh at Wimbledon.

Despite nearly a year away from the game because of a series of health scares, Serena Williams warns that she "wouldn't bet against" herself.

And even though he's yet to drop a set through three matches - something he'd never managed to do at the All England Club - Rafael Nadal keeps talking up his opponent's chances.

As the 125th edition of the grass-court Grand Slam tournament heads into Week 2, all of the principal players are still around, as are the story lines that drew interest at the start, from the Williams sisters' comebacks to the dominance of the leading men.

After Sunday's traditional day of rest at Wimbledon, action resumes today with all 16 men's and women's fourth-round matches. Two stand out in particular: Top-seeded Nadal faces No. 24 Juan Martin del Potro of Argentina, while No. 23 Venus Williams faces No. 32 Tsvetana Pironkova in a rematch of a 2010 quarterfinal won by the Bulgarian.

The Williams sisters have combined to win nine of the past 11 Wimbledon singles titles, and while Serena has played only five matches in the past 11? months, and Venus missed about five months with a hip injury, both are clearly capable of producing top-level tennis.

"Yeah, I'm still alive, and it feels good," said Serena Williams, who could become the first woman since Steffi Graf in 1991-93 to win Wimbledon three years in a row. "You know, I'm hoping to be around - and planning to be around - a lot longer."

Also in the picture are top-seeded Caroline Wozniacki, hoping for her first Grand Slam title, and 2004 Wimbledon champion Maria Sharapova.

"Well, there's no doubt you have to improve with the second week coming. I think that's always the toughest part," said Sharapova, who's won three major titles but none since 2008. "That's where you hope you raise your level."

The top four men - Nadal, No. 2 Novak Djokovic, No. 3 Federer and No. 4 Andy Murray - filled out the semifinals at the French Open, and no one would be too surprised if they did that again at Wimbledon. They lost a total of three sets during Week 1.

Djokovic and Murray are two-time semifinalists at Wimbledon, but neither has been to the final. Murray hopes to give Britain its first male champion at the All England Club - well, at any Grand Slam site, actually - since 1936.

Djokovic, whose 43-match winning streak ended with a loss to Federer in the French Open semifinals, takes on No. 19 Michael Llodra, at 31 the oldest man left and finally in the fourth round on his 11th appearance at Wimbledon. Murray meets No. 17 Richard Gasquet, while the last American man in the field, No. 10 Mardy Fish, plays 2010 runner-up Tomas Berdych.

One thing to keep an eye on Monday: Nadal's fitness. He slipped behind the baseline a couple of times during his third-round victory over Gilles Muller, and afterward acknowledged he was bothered by a right leg muscle. But he also said he can "play with that, without problems."

After so many years of dominance on clay - he won his sixth French Open championship this month - Nadal has become quite good on grass. Since the start of Wimbledon in 2006, he is 29-2 at the tournament. His only losses were to Federer in the 2006 and 2007 finals; Nadal won the 2008 and 2010 titles, and was sidelined in 2009 by tendinitis in his knees.

"Against him, you should play unbelievable tennis. You should play everything perfect, and then maybe you have a little chance," said del Potro, who was forced to play every day from Monday through Saturday because of rain. "He will be the big favorite."

That might be true. But even though del Potro lost in the second round in all three previous visits to the All England Club, his chances against Nadal shouldn't be dismissed.

After all, at the 2009 U.S. Open, del Potro did beat Nadal in the semifinals, then Federer in the final - the only time someone beat both in the course of a single Grand Slam tournament. It's also the only time in the past 25 major tournaments that a man other than Federer, Nadal or Djokovic took home the title.

Del Potro is working his way back after missing most of the 2010 season because of surgery on his right wrist, and while he's seeded 24th, Nadal pointed out that "his level is much better than what the ranking says today."

Federer, who plays No. 18 Mikhail Youzhny on Monday, won his record-extending 16th major title at the 2010 Australian Open.

"I don't go through days thinking, like, 'My God, I haven't won a Grand Slam in so long.' It hasn't been that long, to be honest," said Federer, whose six Wimbledon titles are one behind Willie Renshaw (who played in the 1800s) and Pete Sampras.

But Federer also made sure to note: "I'm always hungry. And that's a good thing."

Women's matchups include No. 7 Serena Williams against No. 9 Marion Bartoli, the 2007 runner-up; Wozniacki vs. No. 24 Dominika Cibulkova; and No. 5 Sharapova vs. No. 20 Peng Shuai of China.

Venus Williams' straight-set loss against Pironkova a year ago represented only the third time in the American's past 11 trips to Wimbledon that she didn't reach the final.

"Unfortunately, I didn't play that well. On a competitive level, I don't feel I competed well. Regardless of how I play, I know I'll be competing this time," said the older Williams sister, who has won five of her seven career Grand Slam singles titles at the All England Club.

"Last year, I let a few games get away, and instead of coming back, I let it all get to me. You can't do that, especially in a Wimbledon quarterfinal," she continued. "I won't let anything get to me this time."

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