NEW YORK — Frances Tiafoe and Taylor Fritz set up an all-American semifinal at the US Open with victories Tuesday, guaranteeing the United States a man in the title match at the country’s Grand Slam tournament for the first time in 18 years.

The 20th-seeded Tiafoe made it to the final four at Flushing Meadows for the second time in three years when his quarterfinal opponent, Grigor Dimitrov, stopped playing because of an injury in the fourth set. Tiafoe was leading 6-3, 6-7 (5), 6-3, 4-1 with midnight approaching when Dimitrov retired from the match, providing an anticlimactic ending to a contest that was not necessarily of the highest quality.

“It’s not the way I want to get through,” Tiafoe said, “but obviously happy to get through. Another semifinal here. Incredible.”

Just one glimpse of the sort of uneven play from both: Dimitrov held three set points in the tiebreaker at 6-3. On the first, he double-faulted. On the second, he double-faulted again. On the third, Tiafoe double-faulted, handing over the set.

Hours earlier, Fritz advanced in a far more satisfying way. After years of climbing the rankings, of becoming the top American man in tennis, of coming close to making a breakthrough at one of his sport’s four most important events, Fritz finally came through at home, beating No. 4 Alexander Zverev 7-6 (2), 3-6, 6-4, 7-6 (3) to reach a major semifinal for the first time.

The 12th-seeded Fritz, a 26-year-old from California, entered the day with an 0-4 record in Slam quarterfinals.

On Friday, he will take on longtime friend Tiafoe, a 26-year-old from Maryland who lost to eventual champion Carlos Alcaraz in the 2022 US Open semifinals.

“It’s the biggest match of me and Taylor’s life. We’ve known each other for so long. I’ve been playing against him since [14-and-under tournaments],” Tiafoe said during an on-court interview. “So to be able to play him here … is going to be awesome. I know we’re two Americans, but I hope you’re all with me come Friday.”

Fritz holds a 6-1 head-to-head edge over Tiafoe as pros.

“I definitely wasn’t thinking he was going to do what he’s done,” Tiafoe said. “He’s changed his body unbelievably. He’s just really talented. He was like a part-time player. He played a couple times a week. And he’ll tell you, he went to a normal high school. He was like a normal kid while we’re out there grinding. Then he started putting more into it.

“As we got closer to it, and seeing how committed he is and how much he wanted it, once we all trend pro, we all just pushed each other to kind of want to become great. Sometimes unspoken, sometimes we speak about it, but nobody wants to leave each other behind. And it’s been a special thing to be part of.”

The Fritz-Tiafoe showdown — “That could be crazy,” Fritz said, before he knew whom he’d face next — is the first semifinal between a pair of American men at any major since 2005, when Andre Agassi defeated Robby Ginepri in New York.

No American man has won a Grand Slam singles trophy since Andy Roddick triumphed at the US Open in 2003; Roddick was the last man from the United States in the US Open final, losing to Roger Federer in 2006. Roddick also was the last man from the United States in a major final, at Wimbledon in 2009.

“We spoke about it for years: This is the group,” Tiafoe said, referencing himself, Fritz, Tommy Paul and Reilly Opelka. “We’ve all been knocking on the door. Taylor has been in and out of the top 10, myself was 10 this time last year and Tommy’s knocking on the door from quarters playing great. It’s only a matter of time.

“And the game itself, it’s not like we’re at a point where you make a quarterfinal, you play [Rafael Nadal] and you’re looking at flights. That’s just the reality. Now it’s totally different. Like, no one’s unbeatable. Especially later in the season, where guys are maybe a little bit cooked, not as fresh and are vulnerable. It’s pretty exciting.”

Dimitrov, who was seeded No. 9, had played a five-setter in the fourth round and appeared to be fading late in the third set against Tiafoe, grabbing at his left hamstring, walking gingerly between points and hitting serves much slower than earlier in the evening. After that set, Dimitrov was visited by a trainer and then headed to the locker room for treatment.

He returned to the court for the start of the fourth set but wasn’t able to move properly and eventually quit. Dimitrov, a 33-year-old from Bulgaria who has appeared in three major semifinals, wouldn’t say afterward exactly what was wrong, only that it was an accumulation of things.

“Just a disappointing moment for me,” Dimitrov said. “I need to reassess a couple of things.”

The other men’s quarterfinals will be played Wednesday: No. 1 Jannik Sinner against No. 5 Daniil Medvedev, and No. 10 Alex De Minaur against No. 25 Jack Draper.

The Associated Press and ESPN’s Coley Harvey contributed to this report.