New York - As Donald Young walked out of Arthur Ashe Stadium - a member of a Grand Slam tournament's final 16 for the first time - U.S. Davis Cup captain Jim Courier held up two thumbs and called out to the 22-year-old American.

"Hey, Donald!" yelled Courier, who won four Grand Slam titles and was ranked No. 1. "Come on, man! Keep up that mojo!"

Since Young turned pro as a teen, there have been all manner of ups and downs, including a rather public spat with the U.S. Tennis Association this year. What's never really been questioned is his skill when a tennis racket is in his left hand, and Young is living up to his potential at this U.S. Open - helping give the United States four men in the fourth round for the first time since 2003.

"It feels like a big corner's been turned, and I really hope it is. I hope he can take this momentum - however it goes here - and use it," Courier said. "It's not been as smooth a road as maybe some pictured for Donald, but he's showing what he can do now."

Cheered on by a raucous, partisan crowd in the Grandstand at Flushing Meadows, Young turned in his second consecutive upset of a seeded player by beating No. 24 Juan Ignacio Chela of Argentina 7-5, 6-4, 6-3 Sunday, two days after eliminating No. 14 Stanislas Wawrinka of Switzerland.

"You have your highs and lows in tennis," said Young, who was born in Chicago and now is based in Atlanta. "I've definitely had the lows. Hopefully I'll have a lot more highs."

One low came in April, when Young wrote on Twitter an expletive-laden rip of the USTA about what he perceived as a snub in its awarding of a wild-card berth for the French Open. Patrick McEnroe, head of the USTA's development program, responded by holding a conference call with reporters to demand an apology.

Both Young and McEnroe insist they've moved on.

After Young's 4-hour, 20-minute victory over Wawrinka, McEnroe tweeted: "In tennis terms Donald Young became a man today." Young, meanwhile, spoke at length Sunday about the help he got at the USTA training center in Carson, Calif., when he spent about four weeks there during the offseason in November and December, crediting that with a vastly improved fitness level that's contributed to his U.S. Open victories.

Asked about the Twitter episode, Young's father, Donald Young Sr., called the whole thing "probably something that had to happen" and noted he wasn't bothered by his son's intent in April, only the way the message was worded.

A loser in his only previous third-round match at a major tournament, the 84th-ranked Young will get to the quarterfinals if he can get past his next opponent, No. 4 Andy Murray of Britain.

"He surprised me," Chela said. "But he came in on a roll. I thought he played a real solid game."

Another American man won Sunday when No. 21 Andy Roddick defeated 81st-ranked Julien Benneteau of France 6-1, 6-4, 7-6 (5). No. 8 Mardy Fish reached the fourth round with a victory Saturday. And No. 28 John Isner became the fourth U.S. man to join the group after beating fellow American Alex Bogomolov Jr., 7-6 (9), 6-4, 6-4.

There haven't been more than four American men in the fourth round since 1995, when eight did it.

Roddick, the 2003 U.S. Open champion, thinks it's probably not a coincidence that there is a bit of a surge at this tournament, because success by one American pushes others.

"There's a healthy jealousy going on right now, which is good. It's only going to help. If Donald Young sees (19-year-old American) Ryan Harrison play well in the summer, he's going to not want to fall behind," Roddick explained. "I think that's what you saw with that 'golden generation' is that they were able to push each other."

Courier was part of that generation, as were Pete Sampras (14 Grand Slam titles), Andre Agassi (eight) and Michael Chang (one).

"It's a great thing to see. You just feel there's a little bit of momentum. There is a bit of a snowball effect at times if it goes the right way," said Roddick, who will play No. 5 David Ferrer of Spain for a quarterfinal berth. "Seems like there's some of that right now."

In 2005, Young became the youngest boy - at 16 years, 5 months - to finish a season as the world's top-ranked junior player.

There were those who talked of a bright, bright future.

"We never had any expectations. Those were other people's expectations. We just wanted to help him do what he wanted to do: become a pro," Donald Young Sr. said Sunday. "It's very exciting that he's doing it at his time and his pace."