Novak Djokovic outclassed Joao Sousa in straight sets to move into the U.S. Open quarters.
NEW YORK — Facing much more resistance from the 90 degree heat and 50 percent humidity than his outclassed opponent, Novak Djokovic figures 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.
''I'm not 21 anymore. That was 10 years ago. I still don't feel old. But at the same time, there is a little biological clock that is not really working in your favor,'' the No. 6-seeded Djokovic told the crowd afterward. ''Sometimes, you just have to survive.''
He reached the quarterfinals for an 11th consecutive appearance in New York as he bids for a third U.S. Open championship and 14th Grand Slam trophy. To add to his resume, though, he might need to beat Federer, who has won five of his men's-record 20 major titles at Flushing Meadows.
Federer was scheduled to play 55th-ranked John Millman of Australia in the fourth round on Monday night. Like Sousa, Millman had never before made it this far at a Slam.
The other quarterfinal on the bottom half of the draw will be 2014 runner-up Kei Nishikori against 2014 champion Marin Cilic or No. 10 David Goffin. Nishikori advanced in the afternoon with a 6-3, 6-2, 7-5 victory over Philipp Kohlschreiber.
Djokovic, who is coming off a title at Wimbledon, wasn't at the U.S. Open a year ago, when he missed the last half of the season because of an injured right elbow that eventually was surgically repaired this February.
Asked whether he thought during that time away about returning to the height of his powers, Djokovic replied: ''I have imagined. I have hoped for. I have prayed for that.''
He improved to 28-0 at the U.S. Open against opponents ranked outside the top 50, and here's another reason it wasn't all that surprising the way things went Monday: Djokovic is now 5-0 against Sousa, taking all 14 sets they've played against each other.
The heat, though, is much tougher on Djokovic, who showed the same blank expression, rosy cheeks and sweat-soaked shirt as during his first-round match last week. That was the first time in tournament history that the U.S. Open created an extreme heat policy for men's matches - they can opt for a 10-minute break between the third and fourth sets - similar to what's standard on the women's tour, when there can be a delay between the second and third sets.
There was that sort of respite during No. 20 Naomi Osaka's 6-3, 2-6, 6-4 win against No. 26 Aryna Sabalenka, a matchup between two powerful players seeking a first Grand Slam quarterfinal appearance. The 20-year-old Osaka was already the first Japanese woman to get to the fourth round in New York since 2004.
Also into the quarterfinals: 2017 runner-up Madison Keys, who overwhelmed No. 29 Dominika Cibulkova 6-1, 6-3 thanks to a 25-7 edge in winners.