Aryna Sabalenka battled back from a disastrous start to defeat Madison Keys in their US Open semi-final on Thursday evening.

The new world number one suffered a bagel in the first set and was a break down twice in the second but produced a spectacular turnaround to prevail 0-6 7-6[1] 7-6[5], thereby denying Keys an all-American final with Coco Gauff.

Earlier in the day, Gauff reached her first Flushing Meadows showpiece match with a 6-4 7-5 win over Karolina Muchova, in a match that was halted for almost 50 minutes due to climate protestors.

Thanks to a succession of winners and several unforced errors from Sabalenka, an inspired Keys remarkably won the opening six games of the match as Sabalenka dropped her first set at the US Open with a whimper.

The Australian Open champion was far from her best and did not hide her frustrations after losing her serve twice in the second set, although she quickly levelled at 2-2 before breaking to love when Keys was serving for the match.

A ruthless tie-breaker display saw Sabalenka force a third set, where Keys once again drew first blood with a break for a 4-2 lead, but the Belarusian underlined her Grand Slam credentials with another stellar fightback before completing the job in the final first-to-10 tie-breaker.

While not without a gruelling effort, Sabalenka banished the memories of two successive semi-final exits to reach her maiden US Open final, where she will face another first-timer in home favourite Gauff.

On the court, the clock showed two hours and three minutes as Gauff dispatched 10th seed Muchova in straight sets, and the opener would seemingly be a formality as the American earned the double break for a 5-1 lead.

Muchova did not give up the ghost and cut the deficit to 4-5, but she was broken to love while serving to stay in the set, and Gauff had taken a 1-0 lead in the second set before protestor chaos began.

Four members of the crowd at the Arthur Ashe Stadium donning shirts with the message "end fossil fuels" stood up and began chanting, and one of them had glued their feet to the floor of the seating bowl.

Play was suspended for 49 minutes before police managed to escort the fourth member of the troupe out of the stadium, after which Muchova and Gauff exchanged seven holds before the latter broke for a 5-3 lead thanks to unforced errors and a double fault from the 10th seed.

Once again, Muchova found a second wind in the face of elimination and broke Gauff while she was serving for the match, and the teenager saw five match points come and go before a backhand unforced error from the Czech proved costly.

Prior to Sabalenka and Gauff's showdown on Saturday, the latter's compatriot Ben Shelton will meet Novak Djokovic for a shot at glory in the men's final against either Carlos Alcaraz or Daniil Medvedev.