Karsten Moran for The New York Times
Gaël Monfils bent over at the waist, one hand on his knee and the other resting on the handle of his racket, which supported him like a cane as he tried to catch his breath inside a stifling Arthur Ashe Stadium.
For days the weather at the United States Open had been unusually cool and dry, but humidity rolled back in on Wednesday, testing the fortitude and resolve of the 33-year-old Monfils and his 23-year-old opponent, Matteo Berrettini, as they traded roundhouse punches for nearly four hours of exhausting tennis.