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Roger Federer, one of the world’s greatest tennis players, may have become an unwitting spokesman for the effects of climate change on Monday at the U.S. Open.

Federer, who is ranked No. 2, seemed to struggle all night in the heat and humidity at Arthur Ashe Stadium, losing in a fourth-round upset to John Millman, an Australian ranked 55th.

“It was hot,” Federer said. It “was just one of those nights where I guess I felt I couldn’t get air; there was no circulation at all.”

This was the first time Federer, who won the U.S. Open five consecutive times from 2004 to 2008, lost to a player outside the top 50 at the tournament.