Alexander Zverev has shown he can win run-of-the-mill tournaments and Masters titles, too. He's shown he can make it to the second week of a major.

What everyone is watching — and waiting — for now is a Grand Slam semifinal, final or trophy.

"Sascha Zverev," said Citi Open co-founder and chairman Donald Dell, using Zverev's nickname after the 21-year-old German won Washington's hard-court tuneup for the U.S. Open a second consecutive year, "is the future of pro tennis."

Zverev is seeded No. 4 at Flushing Meadows, where play begins Monday, and is widely considered the likeliest member of the latest generation of tennis pros to make a deep run at this U.S. Open after getting to his first major quarterfinal at the French Open. Zverev isn't alone, though. He's part of a crop of youngsters who might be ready to take over the sport from the old hands who have dominated it for more than a decade.

Stop us if you've heard that before, though.

"They're still there," Zverev said about the so-called Big Four of Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray. "Obviously, they're still contenders for every single tournament they play."

It's worth noting that Zverev is one of only five active players who's won at least three Masters events. The others? Federer, Nadal, Djokovic and Murray, who are all in their 30s and have combined to win 49 of the past 54 Grand Slam titles.

Men's tennis just keeps skewing older: Last month at Wimbledon, all four semifinalists were in their 30s, the first time that happened at any Slam in the half-century of professional tennis.

Ah, but look closely, and there are signs that change could be on the way.

"They're definitely knocking on that door," Federer said, "and there is some exciting talent around."

At the Citi Open this month, for example, Zverev's victory over 19-year-old Alex de Minaur of Australia made for the youngest final on the ATP World Tour since a 20-year-old Nadal beat a 19-year-old Djokovic at Indian Wells in 2007.

And the ages of the two losing semifinalists in Washington? Andrey Rublev is 20; Stefanos Tsitsipas turned 20 the following week at the Toronto Masters, where he became the youngest player to beat four top-10 opponents at one tournament since the ATP World Tour was established in 1990.

"Four 'NextGen' players in the semifinals. That's amazing for tennis, I think," Zverev said in Washington, referring to the marketing campaign the tour uses to promote up-and-comers. "Me being the oldest — that never happened to me before. It's interesting. And I like where tennis is going. I like the development of the other young guys. It's going to be interesting to see what it'll be like in the future."

Agreed. Zverev is one of seven men who are 21 or younger and ranked in the top 50.

Here's a look at the other half-dozen, each worth keeping an eye on during the U.S. Open:

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STEFANOS TSITSIPAS

Country: Greece

Age: 20

Ranked: 15th (career high)

Plays: Right-handed; one-handed backhand

Career Titles: Zero

2018 Record: 30-20

Best Grand Slam Showing: 4th Round at Wimbledon in 2018

Best U.S. Open Showing: Making debut

His Words: "I am part of a group of (young) players that make me better — and I make them better. We have a very good competition among us. Without this, I probably wouldn't even be inside the top 100."

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BORNA CORIC

Country: Croatia

Age: 21

Ranked: 20th (career high)

Plays: Right-handed; two-handed backhand

Career Titles: 2

2018 Record: 26-14

Best Grand Slam Showing: 3rd Round, four times

Best U.S. Open Showing: 3rd Round in 2017

His Words: "I learned by now that this is tennis and, you know, one week can be great; another one can be a disaster."

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DENIS SHAPOVALOV

Country: Canada

Age: 19

Ranked: 28th (career high is 23rd)

Plays: Left-handed; one-handed backhand

Career Titles: Zero

2018 Record: 25-20

Best Grand Slam Showing: 4th Round at U.S. Open in 2017

Best U.S. Open Showing: 2017

His Words: "I'm only 19 and I've proved a lot to myself this year."

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ANDREY RUBLEV

Country: Russia

Age: 20

Ranked: 37th (career high is 31st)

Plays: Right-handed; two-handed backhand

Career Titles: 1

2018 Record: 15-14

Best Grand Slam Showing: Quarterfinals at U.S. Open in 2017

Best U.S. Open Showing: 2017

His Words: "When you see, for example, somebody, a young guy, winning a big match, I start to think: 'If he can win, maybe I also can win it. Why not?' Is (giving) me more motivation."

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FRANCES TIAFOE

Country: United States

Age: 20

Ranked: 42nd (career high is 38th)

Plays: Right-handed; two-handed backhand

Career Titles: 1

2018 Record: 24-16

Best Grand Slam Showing: 3rd Round of Wimbledon in 2018

Best U.S. Open Showing: 0-3 record

His Words: "There's so much more work that needs to be done to be at the top of the game. I'm at the middle grounds now. I just want to do more. I want to keep working."

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ALEX DE MINAUR

Country: Australia

Age: 19

Ranked: 43rd (career high)

Plays: Right-handed; two-handed backhand

Career Titles: Zero

2018 Record: 16-13

Best Grand Slam Showing: 3rd Round of Wimbledon in 2018

Best U.S. Open Showing: 0-1 record

His Words: "I really wanted to be known in the locker room as that guy that's never going to give up: He's going to find until the end and you're really going to have to play well to beat him. That's something that I've tried to do every time I step out on court."