The U.S. Open semifinal match on Thursday night between Coco Gauff and Karolina Muchova was delayed by more than 40 minutes early in the second set by a group of environmental protesters in the upper levels of Arthur Ashe Stadium.

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 was leading, 6-4, 1-0, and both players left the court.

About 38 minutes into the delay, the U.S. Open said in a post on X, formerly known as Twitter, that the New York City Police Department was “in the process of resolving a fan disturbance.”

“Play will resume as soon as possible,” the U.S. Open said. “Thank you for your patience.”

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

“As we witnessed we had environment protesters up in the loge area,” the tournament director, Stacey Allaster, told ESPN in an interview near the court. “There were three. Two were removed. They quietly left. When security got there they found one of the protesters had physically glued himself, his bare feet, to the cement floor.”

Kurt Streeter and Matthew Futterman contributed reporting.