Protesters chanting 'Stop fossil fuels" forced the US Open women's semi-final between Coco Gauff and Karolina Muchova to be suspended.
Three people stood shouting in the Arthur Ashe Stadium upper tier before the second game of the second set.
With security unable to move the protestors, the players went off court and returned to the locker room.
Play was stopped at 20:05 local time in New York and delayed for more than 25 minutes.
"There is a temporary stoppage as we try to resolve an issue in the crowd," an announcement told the crowd.
"We hope to start play shortly.