Sometimes it feels like the U.S. Open was still being played in Forest Hills when Frances Tiafoe was first pegged as the long-awaited next American star.

Soon, however, the future could finally be here.

The 21-year-old from Maryland cruised to a first-round win Tuesday for the second straight year in Flushing, topping 40-year-old Ivo Karlovic — who retired with back issues, while trailing 6-2, 6-3, 1-2 — on Court 17. Tiafoe’s second career win at the U.S. Open marked his first Grand Slam victory since making a career-best run to the quarterfinals at this year’s Australian Open.

“I’ve had some big results and a lot of quiet results. It’s been up and down for me,” Tiafoe said. “But ultimately I’ve had some highs, and I know how it feels to go deep in these events.”

In joining fellow 21-year-old American Reilly Opelka in the second round, Tiafoe will next face sixth-seeded Alexander Zverev, who squeaked by Radu Albot, 6-1, 6-3, 3-6, 4-6, 6-2. Zverev has won three of their four career meetings, though Tiafoe won their most recent outdoor hard-court battle in three sets in Cincinnati in 2017.

Tiafoe, who is playing in the U.S. Open main draw for the fifth time, made his most memorable appearance in Queens when he dropped a five-set first-round thriller to Roger Federer in 2017. Last year, he lost to Australian Alex de Minaur in the second round.

“I love coming here and playing in New York. I think the energy’s probably the best of all the slams,” Tiafoe said. “The atmosphere’s always unbelievable here. I love coming back to New York and I hope to have a good run here.”

Tiafoe’s most recent appearance in the area ended with him being upset by unknown Jason Jung in the second round of the New York Open at Nassau Coliseum in February. Tiafoe’s return to the U.S. Open showcased the potential that has been hyped since he was a teenager.

Facing one of the toughest serves the sport has ever seen, Tiafoe’s return game was brilliant. He limited the 6-foot-11 Croatian to just six aces, and broke Karlovic on three occasions. Tiafoe, who is ranked 45th in the world, recorded 26 winners and just eight unforced errors.

“It was just one of those days,” Tiafoe said. “I’m just happy to get through, and ready for my next one.”