As Gaël Monfils played in the fourth round of the United States Open on Tuesday, it was impossible to tell that he was winning his match in straight sets, or that he had not dropped a set in the tournament.

He spent most of his time between points muttering to himself in French, often while gesturing to no one in particular with his racket. He argued with the umpire.

On one or two points, he simply stopped playing. He returned one of Grigor Dimitrov’s serves straight into the stands, seemingly on purpose.

The scoreboard, however, told the real story. Monfils beat the No. 7-seeded Dimitrov, 7-5, 7-6 (6), 7-5, lifting himself into the quarterfinals for only the second time in his eight U.S. Open appearances. In the brutal heat of Tuesday afternoon on Ashe Stadium, another straight-sets victory seemed like a gift from above, even if Monfils didn’t appear to enjoy it much.