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 yesterday. “That’s the truth. I’m not lying.’’

After pulling out a 7-6 (8-6), 3-6, 7-6 (7-4), 6-4 victory over 2009 US Open champion Juan Martin del Potro, Nadal said he would get an MRI on the foot and couldn’t be 100 percent sure how fit he’ll be for his quarterfinal tomorrow 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. [Today], 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 (5-7), 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 (7-3), 6-3, 6-2.

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.

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.

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.

After his match, Nadal announced that he will skip Spain’s Davis Cup quarterfinal against the United States in July, blaming his decision on the ITF for poor scheduling.

The six-time French Open champion said there is not enough time between the Davis Cup (July 8-10) and the US Open (Aug. 29-Sept. 11) to properly prepare for both.