Rafael Nadal of Spain hits a return to Juan Martin Del Potro of Argentina fell at the Wimbledon tennis championships in London June 27, 2011. [Photo/Agencies]

Rafael Nadal felt something "crush" in his left foot, and the intense pain immediately made him worry that it might be broken.

And then, as he limped along between first-set points in the fourth round at Wimbledon, came an idea just as scary to contemplate: What if he needed to quit?

"I thought I wouldn't be able to continue playing," the defending champion and top-seeded Nadal said Monday. "That's the truth. I'm not lying."

After pulling out a 7-6 (6), 3-6, 7-6 (4), 6-4 victory over 2009 US Open champion Juan Martin del Potro, Nadal said he would get an MRI exam on the foot and couldn't be 100 percent sure how fit he'll be for his quarterfinal match Wednesday against 10th-seeded Mardy Fish of the United States.

"I'm worried, for sure. I'm going to do the MRI. We'll see what's going on. Tomorrow, we will see," Nadal said. "I cannot predict the future."

By getting past Del Potro, Nadal improved to 30-2 at Wimbledon since the start of the 2006 tournament. He's joined in the quarterfinals by the rest of the top four seeded men: No 2 Novak Djokovic, No 3 Roger Federer and No 4 Andy Murray.

"We've all been playing well, I think the top four or five guys really, for a long time," said Federer, a six-time champion at Wimbledon. "I think it's exciting for tennis."

He dropped a set for the first time in his four matches at Wimbledon this year before coming back to beat 18th-seeded Mikhail Youzhny of Russia 6-7 (5), 6-3, 6-3, 6-3 and reach the quarterfinals at a record 29th consecutive Grand Slam tournament.

Federer, now 11-0 against Youzhny, compiled 54 winners - including 14 aces - and broke six times.

Two-time Australian Open champion Djokovic kept up his bid for a first Wimbledon title by beating Michael Llodra of France 6-3, 6-3, 6-3. Djokovic can replace Nadal at No 1 in the rankings by reaching Sunday's final.

Murray, meanwhile, breezed past No 17 Richard Gasquet of France 7-6 (3), 6-3, 6-2 _ then took a deep bow to the Royal Box, where Prince Wiliam and bride Kate joined the rest of the crowd in giving Britain's top player a standing ovation.

Murray is hoping to become the first British man to win the title at the All England Club since Fred Perry in 1936. After Monday's victory, Murray met with the royal couple, then sheepishly apologized to them for his appearance.

"If I'd known they were coming, I would have shaved," the Scot said with a smile. "I was thinking to myself as I came off I was sweaty and very hairy. I said to them, 'I'm sorry, I'm a bit sweaty.' But it was really nice."

In addition to Nadal-Fish, the quarterfinals will be Djokovic vs 18-year-old qualifier Bernard Tomic of Australia, who beat Xavier Malisse of Belgium 6-1, 7-5, 6-4 to become the youngest man to make it this far at Wimbledon since Boris Becker won a second consecutive championship in 1986; Federer vs No 12 Jo-Wilfried Tsonga of France; and Murray vs unseeded Feliciano Lopez of Spain.

Tsonga got past No 7 David Ferrer of Spain 6-3, 6-4, 7-6 (1), while Lopez came back from two sets down to eliminate Lukasz Kubot of Poland 3-6, 6-7 (5), 7-6 (7), 7-5, 7-5.

In the day's most dramatic match, Nadal's foot began hurting during the first set, particularly when he moved to his left to hit defensive forehands. He was visited twice by a trainer, including right before the tiebreaker, when his foot was taped up, which helped alleviate the problem a bit.

Still, Nadal fell behind 3-0 in the tiebreaker, favoring his left foot during points and stepping gingerly after them. Up in his guest box, Toni Nadal - Rafael's uncle and coach - was gnawing on his finger nails.

Aided by mistakes from Del Potro, Nadal won four consecutive points to go ahead 4-3, but then fell behind 6-5. Nadal saved that set point with a service winner, ended a 20-stroke exchange with a backhand that landed right on the baseline, and took the set when Del Potro double-faulted.

Del Potro - who missed most of 2010 after right wrist surgery - won the second set, but then he tumbled to the turf early in the third, clutching at his left hip. He left to get checked by a trainer, but continued.

Despite both players' health issues, they provided some scintillating tennis. Del Potro pressured Nadal with his powerful forehand and strong serve, often attacking the net. Nadal countered with a series of spectacular passing shots.

After Nadal served out the match at love, the crowd gave both players a lengthy standing ovation.

"I will try my best," Nadal said, "to be very competitive on Wednesday."