The romantic semifinal will have to wait.

Frenchman Gael Monfils had a chance to make it into the men’s final four at the U.S. Open, opposite his girlfriend, Elina Svitolina, who was preparing for Serena Williams in the women’s semis. But the 13th-seeded Monfils instead suffered a brutal defeat Wednesday, dropping a five-set thriller to the 23rd seed, Italian Matteo Berrenttini, 3-6, 6-3, 6-2, 3-6, 7-6 (5).

The match brought the Arthur Ashe Stadium crowd to its feet often, as the afternoon turned to night and the two men slugged it out into a fifth-set tiebreaker. But as Monfils’ final return went long, Berrettini fell to the ground and he became just the second Italian to make it to the U.S. Open semifinals, after Corrado Barazzutti in 1977.

And he turned Monfils into a cheerleader for the rest of the tournament.

“I’m not a sore loser. I gave it my all today,” Monfils said. “I served bad. But it was fun, it was exactly what I play for. I wish I could win, but I love those matches, no matter what.

“I’m happy [and] I’ll be ready to cheer for my girlfriend.”

Berrenttini, a 23-year-old from Rome, had five match points — and needed all of them. In the middle of the fifth set, he was issued a warning for coaching coming from his family box. But with Monfils making 17 double-faults — three in a matter of a couple of minutes late in the fifth set while he was down 6-5 — Berrenttini was able to advance to face the winner of Wednesday night’s match between Rafael Nadal and Diego Schwartzman.

“Maybe one of the best match I ever saw — I was playing, but …” Berrettini said. “The tournament is not finished yet. Be ready for the next match.”

Belinda Bencic continued her roll, as the No. 13-seed from Switzerland followed up her fourth-round defeat of No. 1-seeded Naomi Osaka with a quarterfinal win over No. 23-seed Donna Vekic, 7-5 (5), 6-3.

Bencic will face 19-year-old Bianca Andreescu of Canada in making her first Open semifinal. In the night quarterfinal, Andreescu took out Belgium’s 25th seed Elise Mertens 3-6, 6-2, 6-3.

In the post-match interview, Andreescu, seeded 15th, was asked what she told herself after losing the first set. “Get your s–t together,’’ she said, laughing.

“I think the dream of every tennis player obviously is to win the biggest tournaments,” said Bencic, a 22-year-old in her first Grand Slam semifinal. “I think for sure being No. 1 in the world or winning a Grand Slam is always a dream. But I think it’s still a long way to that.”

— additional reporting by Marc Berman