Even if it turns out to be nothing, which Rafael Nadal insists was the case, the alarming scene in his post-match interview Sunday will go down as memorable.

Answering questions in Spanish, Nadal suddenly started grimacing in pain. He tilted his head back, covered his face with his arm. His face twisted in agony as he slid down in his chair, he motioned for help.

After a few nervous moments with the trainers crowded around the table, Nadal popped back up. A simple leg cramp, he insisted.

"It's bad luck it happened here and not in the locker room," he said.

But because it did, that was the big news Sunday at the U.S. Open -- bigger than Nadal's straight-set win over David Nalbandian that ended about two hours before the cramping episode, bigger than wins by Americans Donald Young (a surprise) and Andy Roddick (not as much of one).

"I just have cramping in my leg, that's all," Nadal said.

His 7-6 (5), 6-1, 7-5 win over Nalbandian was routine, at least as routine as they've been going for the defending champion and No. 2 seed through the first week at Flushing Meadows. He gave up a break early in the first set but fought back to force a tiebreaker that he won. He gave back a break in the third set to turn a possible 6-3 close-out into something much closer.

He is winning but not steamrolling -- serving well, but not dominating the way he did last year. Last year, he lost serve five times in seven matches; this year, he's lost nine service games in three.

"I was happy about almost everything today," Nadal said. "I think my movements worked pretty well, and the forehand worked really well, and the backhand, too. Just when I had to win the match at 5-3, I played a really bad game there. For the rest of the match, I happy about everything."

But shortly after saying that, the cramping came -- in his right quadriceps and his right hamstring. A scary scene with a few dozen reporters looking on and the cameras rolling. Certainly something that will give people plenty to talk about as Nadal gears up for the second week, even if it really wasn't too big a deal.

"Not to put a dampener on the story, which I know you guys think is really big, but people cramp after matches when you're cold," Roddick said after his match. "It's just something that happens. It's just unfortunate it happened in front of you all. Every single player in there has had that happen before. Every single one."

Indeed, there were other players struggling with the heat on a windy, humid day in Queens with temperatures in the mid-80s.

In Louis Armstrong Stadium, No. 26 Flavia Pennetta backed up her victory over Maria Sharapova with a 6-4, 7-6 (6) win over No. 13 Peng Shuai of China. But she also struggled with the heat. Trailing 5-3 in the second, Pennetta got a break to stay in it. She forced a tiebreaker but appeared near exhaustion as the set wore on. The match took 2 hours, 31 minutes.

"This one is one of the worst I've ever felt on the court," Pennetta said.

Pennetta fell behind 5-0 and 6-2 in the tiebreaker but won the last six points to pull out the match. Pennetta made it 6-6 when she answered Peng's overhead with a shot that hit near the frame of her racket for a crosscourt winner.

Trying to move past the quarterfinals in a Grand Slam for the first time, Pennetta will play Angelique Kerber of Germany, a 6-4, 6-3 winner over Monica Niculescu of Romania.

Later, Roddick and Young won their matches.

Roddick, seeded 21st, defeated Julien Benneteau 6-1, 6-4, 7-6 (5) -- a relatively easy match except for a third set that dragged out to 65 minutes.

Roddick is working his way into tournament shape after an injury-filled season. His next match is against No. 5 David Ferrer, who beat 26th-seeded Florian Mayer of Germany 6-1, 6-2, 7-6 (2).

"A lot of what I do feeds into what he does well also," Roddick said of the fellow baseliner. "I don't think there's a lot of secrets between us. Most of the time we played, whoever has executed better has won."

Young, the one-time top-ranked junior in the world, took another step toward fulfilling his vast potential with a 7-5, 6-4, 6-3 upset over No. 24 Juan Ignacio Chela on the Grandstand Court.

The 84th-ranked Young, who also has a win over 14th-seeded Stanislas Wawrinka this week, is in the fourth round of a Grand Slam for the first time.

"It's been tough," he said. "I've been through times I didn't want to play or pick up a racket anymore, didn't want to watch TV because they'd be talking bad about me. I got up this morning and was watching CBS and they were showing good things about me."

Also, 2009 champion Juan Martin del Potro, returning to the U.S. Open after missing last year with an injury, fell 4-6, 7-6 (5), 6-2, 7-6 (3) to No. 12 Gilles Simon.

Playing later Sunday were No. 4 Andy Murray and No. 2 Vera Zvonareva.

And, in an all-American match, No. 28 John Isner was to play Alex Bogomolov Jr.