With her steely eyes and ear-splitting shrieks, Maria Sharapova is regarded as one of the fiercer competitors in tennis — remorseless in steamrolling inferior opponents and even more determined when clawing out of deficits.

But with her serve and groundstrokes betraying her far too often Friday at the U.S. Open, Sharapova couldn’t summon the tools to dig out of the hole she’d dug for herself against Italy’s Flavia Pennetta.

As a result, Sharapova, the tournament’s No. 3 seed and 2006 champion, became the latest top-10 seed to tumble, with Pennetta advancing to the fourth round with a 6-3, 3-6, 6-4 upset.

If Sharapova’s defeat was the result of a breakdown in confidence, American Donald Young’s five-set triumph over 14th seed Stanislas Wawrinka represented a breakthrough.

From the day he turned pro at 14, Young was hailed as the next great American tennis champion. But when the world-class results and ranking didn’t follow, Young was held up as a cautionary tale.

Flavia Pennetta of Italy returns a shot against Maria Sharapova. (Nick Laham/GETTY IMAGES)

On Friday, with a packed stadium of fans cheering him on, the 22-year-old Young delivered the most impressive victory of his career. It wasn’t simply that he toppled the 14th seed in a Grand Slam tournament. Young, granted a wild-card entry into the event, did it the most difficult way possible — prevailing in a five-set match for the first time and closing with a commanding performance in a tiebreak. The score: 7-6 (9-7), 3-6, 2-6, 6-3, 7-6 (7-1).

“To come through a match like that and not wilt like I have before, it’s great,” said Young, still smiling well after the 4-hour, 20-minute marathon on Court 17.

Patrick McEnroe, head of the U.S. Tennis Association’s Professional Development Program, tweeted in exultation: “In tennis terms Donald Young became a man today.”

Informed of the comment later, Young joked: “I thought I became a man, you know, when I turned 21. On a serious note, that’s great for someone of his status to say something like that and really feel that. It’s great.”

Young is one of five American men to reach the U.S. Open’s third round.

In Friday night’s featured match, 29-year-old Andy Roddick made clear that he’s not ready to cede his place in the sport to the next generation of challengers, turning back 18-year-old Jack Sock, 6-3, 6-3, 6-4, in a duel of former U.S. Open junior boys’ champions from Nebraska.

Roddick, who won the junior title in 2000, entered the match with a 580-192 career record in the top touring series; Sock, the 2010 boys’ champion, was 1-2.

But Sock showed no trace of nerves, calling the chance to play his idol in the world’s biggest tennis venue “the best tennis experience of my life.”

“I was out there just loving every minute of it and every point,” Sock said.

While Sock shares Roddick’s weapons — a huge serve and massive forehand — Roddick was superior at both and had the composure to stay in rallies longer, finishing with 13 unforced errors to Sock’s 38.

After shaking hands at the net, Roddick invited Sock to his house in Austin to practice in the offseason.

“I’d have never thought I’d play another guy from Nebraska in my career,” Roddick said. “It was just cool. I could draw so many parallels to what he was going through.”

Earlier Friday, John Isner dispatched fellow American Robby Ginepri, 6-4, 6-3, 6-4. He’ll next face fellow American Alex Bogomolov Jr., who ousted Rogerio Dutra Da Silva, 6-4, 6-3, 7-5.

The two American women in action struggled against higher seeds. New Jersey’s Christina McHale, 19, chided herself for being too passive against 25th seed Maria Kirilenko, who fought off leg cramps to advance, 6-2, 6-3.

Sharapova’s loss was the day’s biggest surprise.

The tone was set early, with Sharapova double-faulting twice to get broken in the opening game. Pennetta bolted to a 4-0 lead and won the opening set.

Sharapova stormed back to claim next set. But instead of building on the momentum, she fell behind 1-4. Serving a must-win game at 4-5, Sharapova double-faulted on the first two points. And though she insisted that she never questioned her ability to come back regardless of the score, her body language and face told a different story, stricken with tension and self-doubt.

She finished with 12 double faults and 60 unforced errors.

“It’s quite a liability,” former pro Mary Joe Fernandez, who called the match for ESPN, said in an interview afterward when asked about Sharapova’s serve. “Her mind is becoming obsessed with that shot, even though I don’t think that’s what hurt her today. Today it was just the continuation of these unforced errors off the ground, but I think it’s because her focus is so much on the serve and worrying about it.”

Also advancing: Defending champion Rafael Nadal, whose opponent, Nicolas Mahut, retired with an apparent injury after the Spaniard got out to a 6-2, 6-2 lead; and fourth-seed Andy Murray, who survived a scare from Robin Haase before prevailing, 6-7 (7-5), 2-6, 6-2, 6-0, 6-4.