A protester glued his bare feet to the concrete in the stands of the US Open on Thursday night ... and it caused such a commotion, it delayed Coco Gauff's huge semifinal match for nearly an hour.
The bizarre scene unfolded just minutes into Gauff's tilt with Karolina Muchova at Arthur Ashe Stadium in New York ... when a quartet of environmental activists created a massive ruckus in the crowd.
The four, wearing tees with the words "End Fossil Fuels" printed on them, began chanting, leading authorities to quickly spring into action.