The U.S. Open semifinal match on Thursday night between Coco Gauff and Karolina Muchova was delayed by 49 minutes early in the second set by four environmental protesters in the upper levels of Arthur Ashe Stadium who were calling for an end to fossil fuels. One protester glued his bare feet to the ground.

The protest confused fans, television commentators and the players themselves, who were trying to understand what the group was protesting and why the match had been delayed so long. When play stopped, Gauff, the eventual winner, was leading, 6-4, 1-0. Both players left the court.

As stadium security tried to remove the protesters from the stadium, at least 10 New York City police officers were seen surrounding the disturbance in the loge level.

Chris Widmaier, a spokesman for the United States Tennis Association, which hosts the U.S. Open, said after the match that three of the four protesters were escorted out of the stadium without any issues. But police officers and medical personnel were brought in to safely remove the fourth protester who had affixed his feet to the cement floor with some sort of product, Widmaier said.

“We plan for it,” Widmaier said. “We prepare for a lot of things. To my knowledge, this kind of protest seems to be happening at other places. We are very aware of environmental protests. It happened at Wimbledon. It happened at the Citi Open.”

The U.S. Open had a designated area for protests outside the Billie Jean King National Tennis Center, so long as groups have a permit, Widmaier said.

A spokesman for the New York Police Department said four protesters inside Arthur Ashe Stadium were taken into police custody “without incident.”

It was unclear whether the police had charged the demonstrators. The protest was still under investigation late Thursday night, the spokesman said.

It was also unclear how security and the police officers were able to unstick the man’s feet from the stadium floor.

As the delay went on, several people in the crowd were heard chanting, “Kick them out,” referring to the protesters.

Given the history of similar protests at tennis matches, Gauff told reporters after the match that she had a feeling there would be a protest at the U.S. Open. Gauff said she didn’t know exactly what the protesters were calling for, but added that she believes in climate change.

“I think there’s things that we can do better,” Gauff said. “But I prefer it not happening in my match.”

Muchova said after her loss that the delay “obviously changed the rhythm” of the match.

“It is what it is,” Muchova said. “What can we do about it?”

At about 8:50 p.m., about 45 minutes into the delay, the players returned to the court to warm up.

This was not the first instance of an environmental protest at a major tennis tournament this year. At Wimbledon in July, environmental protesters halted play during a match by throwing confetti onto a grass court. In 2022 at the Rod Laver Cup in London, a protester set fire to his arm after running onto the court during a match, briefly setting fire to the playing surface.

Extinction Rebellion NYC, an environmental activist group, said in a statement after the delay that its activists were there to call for an end to fossil fuels, and that there is “no tennis on a dead planet.”

Miles Grant, an Extinction Rebellion spokesman, said in a phone interview that the protesters at the U.S. Open were safely escorted off the tennis grounds.

“They were not hurt,” he said. “That was really important to us.”

Grant, who was not at the U.S. Open on Thursday, said in an earlier statement that “the climate is already more disruptive than any activists can possibly be.”

“Just look at the U.S. Open and other big tennis events — year after year, the average temperatures have been rising, making it hotter and more dangerous for the players and spectators,” Grant said. “At some point, there will be fewer outdoor sporting events due to excessive heat.”

The group’s protest came as players at the U.S. Open have been forced to contend with some of the hottest weather of the tournament, with temperatures this week rising into the 90s and humidity making it feel even hotter at times. Highs in New York this week have been about 10 degrees above normal for this time of the year, according to the National Weather Service.

Nicole Andersen, a nutritionist from Brooklyn, was sitting about 12 rows behind the protesters in Section 114 of the loge level. Initially, Andersen said she thought they were cheering loudly for Gauff.

“Then we realized it was some kind of protest,” Andersen said. “Then they would not shut up and stop.”

Andersen said that climate issues are “certainly a problem,” but added that the protesters at the match may have chosen “not the most effective way to protest for change.”

During the delay, Gauff and Muchova tried to stay warm and loose in the locker room and the warm-up area. Muchova got a massage and jogged lightly in the hallway. Gauff talked with tournaments workers, leaning over to see pictures of the protesters circulating on social media.

On the broadcast, Gauff could be heard telling her coaches that security and police were “negotiating” with the protesters, “like it’s a hostage situation.”

Gauff’s matches during the tournament have drawn many boldface names and Thursday night was no exception. Lin-Manuel Miranda, the Broadway writer and performer, Julius Randle, the Knicks star forward, and Naomi Osaka, the four-time Grand Slam singles champion, like all in attendance, got more than a tennis match.

Gauff will now play the winner of the other semifinal matchup, between Aryna Sabalenka and Madison Keys, who were scheduled to play in Ashe Stadium after the first match.

Orlando Mayorquin and Matthew Futterman contributed reporting.