The U.S. Open crowd was in a celebratory mood, with an all-American matchup in the final on the verge of being confirmed.

Then Aryna Sabalenka spoiled the party.

The Belarusian player, seeded second, rallied to beat 17th-seeded Madison Keys 0-6, 7-6 (1), 7-6 (5) in the semifinals at New York on Thursday.

Awaiting Sabalenka in the final will be 19-year-old Coco Gauff, the youngest American woman to reach the U.S. Open final since Serena Williams in 1999.

Gauff, the sixth seed, topped 10th-seeded Karolina Muchova of the Czech Republic 6-4, 7-5 in the first semifinal.

Sabalenka, 25, won the Australian Open championship in January, her first appearance in a major final. Since then, she got to the semifinals at the French Open and Wimbledon before reaching the final in New York.

Gauff is also headed to her second Grand Slam final, having lost to Poland’s Iga Swiatek at the 2022 French Open.

In the late match on Thursday, Keys cruised through the first set in 31 minutes. Sabalenka failed to convert two set points at 5-6 in the second set but quickly shook off the disappointment to dominate the tiebreaker.

Both players traded service breaks midway through the third set, which went to a first-to-10 tiebreaker. Sabalenka celebrated when she won a point to go up 7-3, thinking she had won, forgetting that the final-set tiebreaker is extended.

“I’m really happy that after that reaction I was able to stay focused and I was able to stay focused because … it could be the other way around,” Sabalenka said on court postmatch. “She played incredible tennis. … It was just another level. Somehow, I don’t know how actually I turned around this match and I won it.

“It really means a lot, being in the final at the U.S. Open for the first time, it means a lot to me.”

In the first semifinal, Gauff slammed a winner at the net to end a 40-shot rally for her sixth match point. Muchova’s unforced error on the next point ended the contest.

“I grew up watching this tournament so much, so it means a lot to be in the final,” Gauff said on court in her postmatch interview. “A lot to celebrate, but you know the job is not done so hopefully (the fans) can back me on Saturday.”

The teen added that she wasn’t sure she would watch the Sabalenka-Keys match, saying she might watch some anime instead.

Muchova, who reached her first Grand Slam final in Paris this year, was hurt by committing 36 unforced errors to Gauff’s 25. Muchova’s slight edge in winners, 15-13, wasn’t enough to compensate.

Gauff continued her hot summer. Since losing in the first round at Wimbledon, she won the title in Washington early last month, got to the quarterfinals in Montreal and won the championship in Cincinnati prior to arriving in New York.

Sabalenka said of facing off with Gauff, “She’s an unbelievable top player. She’s playing really incredible tennis here at the U.S. Open. Yeah, the crowd will be really supporting her a lot. It’s something I’m expecting. This is nothing crazy.

“I’ll just go there and just do everything I can. I’ll be there. I’ll be fighting for every point. I’ll do my best.”

The Gauff-Muchova match was delayed more than 45 minutes early in the second set due to a disturbance from environmental protesters in the stands. Four people were arrested, according to the U.S. Tennis Association.

–Field Level Media