Here's a window on the world of elite tennis: Donald Young's career. A prodigy at 15, dog paddling just to stay afloat in the pro game for almost a decade, Exhibit A that the sport -- more physical, more competitive -- no longer is as kid-friendly as it used to be.

Now, at 26, Young is a reminder of the benefits of physical and emotional maturity on tour. He represents hope for -- and has come to be surrounded by -- later-bloomers.

Consider the man Young is playing in Monday's fourth round, Switzerland's Stan Wawrinka, who is 30 and didn't win his first major tournament until he was almost 29.

U.S. OpenU.S. Open: Men's results

Or Young's fellow American in the fourth round, John Isner, who didn't break into the rankings' top 10 until he was 27 and is on the verge of doing so again at 30. Or Isner's opponent Monday night, Roger Federer, 34. "As he gets older,'' Isner said, "he seems to keep getting better."

Gone are the days, tennis insiders agree, when a 19-year-old lad will win the Open men's title, as Pete Sampras did in 1990.

"Look at the guys that are doing well," Young said after his second five-set victory after losing the first two sets in this year's Open. "They're, like, 33. I'm 26."

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There were 10 teenagers in the men's original 164-player draw, but by the round of 16, there were none left, and five still alive were at least 30. Every man in the fourth round was at least Young's age, including defending champ Marin Cilic, who defeated Frenchman Jeremy Chardy in four sets Sunday.

Another Frenchman, Jo-Wilfried Tsonga, also 30, dismissed compatriot Benoit Paire in straight sets.

Young is playing his 26th Grand Slam event, and this is only the second time he has been as far as the fourth round. Twenty-one times he went out in the first or second round. He has spent most of his career ranked lower than 100.

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He is No. 68 now, so his future remains unsettled. But now there is "improvement," he said. "Resilience. I've kind of been beat up. I've beat up myself. I've kind of been down. I've had good times, bad times. But hopefully, it's not over.

He added: "I've just changed me quite a bit.

" . . . I mean, I never really forgot the fact that when I was 15, I was supposed to win Wimbledon the next year. When I was 19, I wasn't going to be good, ever. The results at a young age kind of change the perspective a little bit. That's fine. That's what happens when you kind of do things at an accelerated pace.

"I think every tennis player, every time they lose, they want to quit or do something else, especially after a rough loss. I've had times where I seriously considered it, most of it when I was quite young. But now things are going well. I'm here now. I'm 26. I'm right in the thick of things. That's when a lot of people start to play well. I don't want to go home."

Could it be a weird sort of compliment that Young arrived at his locker Sunday to find his shoes were stolen? Perhaps somebody looking for a souvenir of an up-and-comer? Time will tell if those shoes are worth something.

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Djokovic into quarters. Top seed Novak Djokovic was pushed by No. 23 seed Roberto Bautista Agut but ended up a winner Sunday night. Bautista Agut took the second set and had the crowd on his side, but Djokovic prevailed, 6-3, 4-6, 6-4, 6-3, to reach his 26th consecutive Grand Slam quarterfinal.