The U.S. Open semifinal between Coco Gauff and Karolina Muchova was delayed early in the second set because of a disruption by environmental activists in the Arthur Ashe Stadium stands on Thursday night.

Gauff was ahead 6-4, 1-0 when play was halted for at least 40 minutes.

Tournament referee Jake Garner came out onto the court, while security headed toward where the protesters were. They were wearing shirts that read, “End Fossil Fuels.”

At least one person glued their feet to the ground, making it harder to be removed, a spokesman for the U.S. Tennis Association said.

At least two protesters were taken away. Later, more than a half-dozen police officers arrived -- greeted by cheers from fans near the section where the disruption happened. Spectators were asked to move away to clear a path for the police.

Gauff sat on her sideline bench for a bit, eating fruit out of a plastic container, before then getting some tennis balls and hitting a few practice serves. Muchova was briefly visited by a trainer during the interruption.

Eventually, both players headed toward the locker room as the delay continued. Gauff sat on a treadmill, a towel draped over her lap, while chatting with members of her team.

It is the latest in a recent series of protests at sporting events -- and tennis, in particular -- related to the use of fossil fuels.

At Wimbledon in July, two matches were interrupted when environmental activists jumped out of the stands at Court 18 and scattered orange confetti on the grass.

At a U.S. Open tune-up tournament in Washington last month, about a dozen people were asked to leave the site after chanting and displaying signs protesting the use of fossil fuels.

Gauff, a 19-year-old American, and Muchova, a 27-year-old from the Czech Republic, were both playing in the semifinals at Flushing Meadows for the first time.

Their match was the first of the evening. The other women’s semifinal, between Aryna Sabalenka of Belarus and Madison Keys of the United States, was scheduled to begin after Gauff vs. Muchova finished.

The two winners Thursday will play each other for the women’s singles championship in Ashe on Saturday.