American men completed their strongest first-week showing since 2003 at the U.S. Openon Sunday, with wild-card Donald Young leading the long-awaited charge.

Young, 22, toppled his second seeded opponent of the tournament — ousting No. 24 Juan Ignacio Chela in straight sets — to reach the last 16 of a Grand Slam for the first time in eight years as a pro. Wearing a neon yellow shirt and a smile that was even brighter, Young pumped his fist in triumph, having finally shaken his reputation as the sport’s most gifted underachiever with his performance this week.

The unseeded Young joins No. 8Mardy Fish, No. 21Andy Roddick and No. 28 John Isner, who handed fellow American Alex Bogomolov Jr. a 7-6 (9-7), 6-4, 6-4 defeat, in advancing to the second week.

The last time as many American men got that far in the 128-player field, 21-year-old Roddick — the youngest among the bunch that included Andre Agassi, Todd Martin and Taylor Dent — went on to win the 2003 U.S. Open.

“I think that’s what you saw with that golden generation,” said Roddick, now 29, following a convincing 6-1, 6-4, 7-6 (7-5) victory over Julien Benneteau. “They were able to push each other. It’s a great thing to see.”

Donald Young reacts after winning his match. (Julio Cortez/Associated Press)

But the feel-good story line for the American hosts was overshadowed at least temporarily by a scare involving U.S. Open champion Rafael Nadal, who was seized by such painful cramps in his right leg that he halted his postmatch news conference, unable to continue, and grimaced for several minutes before sliding out of his chair to lie down.

After being treated by ATP trainers, who were summoned as reporters were ushered out, Nadal resumed his news conference about 15 minutes later. “I just have cramping in my leg. That’s all,” he said in brief remarks in English. “I just have cramping in front and behind [gesturing to his right thigh]. That’s why I was so painful. That’s all.”

The episode occurred roughly two hours after Nadal defeated David Nalbandian, 7-6 (7-5), 6-1, 7-5, to advance to the round of 16. The 11 a.m. match on Arthur Ashe Stadium lasted 2 hours 29 minutes (fairly lengthy for a straight-sets victory) and was played under sunny and humid skies, with temperatures in the low 80s.

After being stretched out and offered ice bags, Nadal was helped to his feet and resumed his interview with Spanish journalists, explaining that it was not the first time he had suffered postmatch cramps. What made this bout so debilitating, he said, was the fact that the muscles on the top and back of his right thigh cramped simultaneously, causing intense pain and making it impossible to remedy on his own.

Roddick backed up Nadal’s dismissal of the episode as routine, saying: “People cramp after matches when you’re cold; 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.”

Roddick bolted to a 5-0 lead against a somnolent Benneteau and closing the opening set in 25 minutes. As Roddick’s confidence grew, so did the aggression of his tactics.

His big serve on target, he blasted 21 aces past the Frenchman, ripped 45 winners while committing just 16 unforced errors and won the point on 20 of his 32 forays to the net.

Up next for the 21st seeded Roddick is No. 5 David Ferrer of Spain, who defeated Florian Mayer, 6-1, 6-2, 7-6 (7-2).

Like Roddick, Young got off to a terrific start against Chela, taking a 5-2 lead. After letting the Argentine pull even, Young could easily have folded. In past years, he might have been expected to fold. Instead Young did a bit of venting and played on, closing the set and rolling to the 7-5, 6-4, 6-3 victory in 2 hours 13 minutes.

In second-round action on Friday, Young outplayed and outlasted 14th-seeded Stanislas Wawrinka to claim the first five-set victory of his career.

With each round he wins, Young has reflected a bit on the struggles he faced after turning pro at 14. Almost overnight, he went from being the most promising junior in the world to the most premature touring pro, and his 26-55 career record in ATP matches reflects years spent trying to maintain his confidence while losing far more often than he won.

“I was used to winning,” Young said, referring to dominance of the junior circuit. “When you’re used to winning, and you start losing, it doesn’t feel good. . . . It was a learning experience. I think those things I did helped me now. It’s better late than never.”

Up next for Young: No. 4 Andy Murray, who eased by Spain’s Feliciano Lopez, 6-1, 6-4, 6-2.

In women’s action, No. 9 Samantha Stosur lost the longest recorded women’s singles tiebreak in the history of the majors but rebounded to defeat Maria Kirilenko, 6-2, 6-7 (17-15), 6-3.

Stosur will face No. 2 seed Vera Zvonareva, who defeated Germany’s Sabine Lisicki, 6-2, 6-3.