Taylor Fritz retuns a shot to Frances Tiafoe during the Winston-Salem Open on Aug. 22. (Grant Halverson/Getty Images)

With six teenagers in the men’s top 300 professional tennis rankings, the United States boasts more than any nation — and the most it has had since 1990.

That bumper crop more than a quarter-century ago included some familiar names: Andre Agassi, Michael Chang, Jim Courier and Pete Sampras. Together, they won 27 Grand Slam titles, led by Sampras’s 14. Agassi, Courier and Sampras also reached No. 1.

The latest group of teens to make noise — Taylor Fritz, Jared Donaldson, Frances Tiafoe, Stefan Kozlov, Tommy Paul and Reilly Opelka — are far from household names. But if sheer numbers are any signpost, fortunes are looking up for U.S. men’s tennis with the year’s final major, the U.S. Open, set to begin Monday.

“I’m really optimistic about this group and their potential,” said Martin Blackman, the U.S. Tennis Association’s general manager of player development.

Jared Donaldson serves to Stan Wawrinka of Switzerland at the Western & Southern Open on Aug. 17. (Joe Robbins/Getty Images)

No American man has captured a Grand Slam title since Andy Roddick at the 2003 U.S. Open, the country’s longest major singles drought in history. The highest-ranked American is No. 21 Steve Johnson.

Led by Fritz, 18, who is already inside the top 60, these youngsters have turned outstanding junior careers into early success on the pro circuit — a rarity in today’s physically demanding game, which favors mature minds and developed bodies. All have moved up the rankings in 2016 by dozens, if not hundreds, of spots.

“I think it is a pretty special group,” said 22nd-ranked John Isner, the flagbearer of U.S. men’s tennis in recent years.

Fritz, last year’s U.S. Open junior champion, shot up the rankings on the heels of a few torrid weeks this winter. He reached the Memphis final in just his third ATP Tour-level event and the quarterfinals at Acapulco two weeks later. At No. 54, the lanky Californian is the youngest man in the top 100.

Nineteen-year-old Donaldson, ranked 122nd, has been making steadier progress. He recently reached the fourth round at the Masters 1,000 in Toronto where he notched a win over No. 33 Fabio Fognini.

Tiafoe, 18, who trained at the Junior Tennis Champions Center in College Park, has been racking up victories mostly on the lower-tier Challenger Circuit. The 124th-ranked Maryland native owns wins over Fritz and Donaldson this year.

Frances Tiafoe during his match against Taylor Fritz at the Winston-Salem Open on Aug. 22. (Grant Halverson/Getty Images)

Opelka, 18, has enjoyed a more meteoric rise. The 6-foot-11 Opelka beat No. 28 Kevin Anderson on his way to a semifinal showing at Atlanta this month. It pushed his ranking inside the top 300 after ending last season at No. 975.

Friendly but competitive, this group has traveled in close circles for years. They faced off in juniors and often trained together with the USTA development program. Opelka was the best man at Fritz’s wedding in July.

During their transition to the pros, they have fed off each other’s accomplishments.

Opelka, last year’s Wimbledon boys’ winner, called Fritz’s surge “really motivating.”

“I practice with him,” said No. 295 Opelka, who spent several months recovering from a stress fracture in his right foot last year. “I play with him. When he was around 50 in the world and I was 900, I was playing a high level with him, very close.”

Fritz said his back-and-forth rankings duel with Tiafoe at the end of 2015 kept him eager and hungry.

“I think maybe it’s working the same for them,” Fritz said. “They know that I’m not really any better than they are.”

Blackman agreed. “Because they are so close in age, there is a lot of positive peer pressure,” said Blackman, who took over the USTA’s top development position 15 months ago. “They can look to the left and look to the right and see good results. . . . I think it breeds a lot of belief.”

This cluster of teens is diverse. From the towering Opelka to the explosive Tiafoe to the technically sound Fritz and tennis-smart Kozlov, they possess a wide cross section of skill sets and body types.

One thing in common: None cut their teeth in college.

That route has had mixed results in recent years. Several top prospects that went straight to the pros out of high school a decade ago ended up burned out, injured or out of the game — what Blackman’s predecessor at USTA player development, Patrick McEnroe, once called a “lost generation.”

College seasoning has since regained favor. Both Johnson and Isner attended four years of university before jumping to the pros.

The across-the-board success of these players, however, suggests that they have made the right call.

“They went into it with their eyes open,” Blackman said. “I still believe college is a very important pathway to the professional ranks.”

College or not, peaks and valleys are inevitable.

Fritz has struggled of late, losing in the first round of five of his past seven tournaments.

“I have high expectations for myself, and I want to do better than I’m doing,” he said. “I’m sure people have started to figure out things about me.”

McEnroe, who comments for ESPN, said his goal when he took over the development program in 2009 was to help more players break into the top 100.

“I think that’s about to happen with this group,” he said.

Where it goes beyond that is hard to predict.

“A few of them can be top players,” he added. “None are a lock to win majors.”

The key, said Blackman, is staying “process focused.”

He cited the professionalism, work ethic and improvement in young stars like top-10 player Dominic Thiem, as well as established champions like Novak Djokovic and Andy Murray.

“They are always adding to their games,” Blackman said.

A maiden win at their home Grand Slam is a top goal for all of them. That won’t happen here for Opelka, Kozlov and Paul, who all lost in qualifying. Donaldson beat Santiago Giraldo, 6-2, 6-2, on Friday to qualify for the main draw and will face No. 12 seed David Goffin of Belgium.

Fritz fell in the first round of all three Slams this season and said he is tired of reading that he is the highest-ranked man yet to notch a win at a major.

“I’ve seen it like five times,” he said. “It’s been tough not to notice it for me.”

He will have to go through a compatriot to do it. Fritz faces No. 26 seed Jack Sock in the first round. Tiafoe, who received a wild card, also drew a fellow American. He plays Isner.

Who has the most upside from this sextet? Blackman won’t say.

He relayed a conversation he had with Roddick about the pressure the former No. 1 felt as the leading man of his generation. Roddick came of age with James Blake, Mardy Fish, Robby Ginepri and Taylor Dent.

“He had one piece of advice,” Blackman said. “Don’t anoint one of these guys before they break out.”