NEW YORK — Facing much more resistance from the 90-degree heat and 50-percent humidity than from his outclassed opponent, Novak Djokovic figured he can count on cooler conditions during a night match at the U.S. Open his next time out.

The next foe? That could be Roger Federer.

Djokovic left the court at Arthur Ashe Stadium for a medical timeout — the second time during the tournament he’s sought help from a doctor because of harsh weather — during what would become an otherwise straightforward 6-3, 6-4, 6-3 victory over 68th-ranked Joao Sousa of Portugal on Monday in the fourth round at Flushing Meadows.

Federer was playing 55th-ranked John Millman of Australia in the fourth round late Monday night. Like Sousa, Millman had never before made it this far at a Slam.