Novak Djokovic defeated Joao Sousa in straight sets at the 2018 U.S. Open at Flushing Meadows, as the former world No. 1 prevailed on Monday.
Djokovic was clearly uncomfortable in the match during the extreme heat of the day, but the No. 6 seed won 6-3, 6-4, 6-3 to claim a quarter-final spot.
Madison Keys showed no mercy as she derailed Dominika Cibulkova, conceding only four games to win in straight sets. The American dominated the contest 6-1, 6-3.
Naomi Osaka joins Keys in the next round after an exciting match with Aryna Sabalenka. The 20-year-old Japanese starlet, who is a resident in the United States, claimed the win 6-3, 2-6, 6-4.
There was double delight for Japan as Kei Nishikori also qualified after defeating Philipp Kohlschreiber 6-3, 6-2, 7-5.
Arthur Ashe Stadium
(14) Madison Keys def. (29) Dominika Cibulkova 6-1, 6-3
(6) Novak Djokovic vs. Joao Sousa 6-3, 6-4, 6-3
Louis Armstrong Stadium
(21) Kei Nishikori def. Philipp Kohlschreiber 6-3, 6-2, 7-5
(20) Naomi Osaka def. (26) Aryna Sabalenka 6-3, 2-6, 6-4
Two-time champion Djokovic was economical as he beat Sousa in two hours, but the Serbian found the playing conditions hugely challenging as he suffered with fatigue.
The Serb had to leave the court after the second set for a medical assessment, but the 31-year-old was back in the action in a matter of minutes.
Sousa could not take advantage of his opponent's weakened state, and a better player would have punished Djokovic as he clearly struggled.
Djokovic is starting to show his age in a sport that is getting younger, and he will find it hard to make the final in his current physical condition.
The legend hit nine aces on his way to victory, as the Portuguese made 28 unforced errors as he departed the competition.
George Bellshaw of Metro quoted the winner of the match after the conclusion:
George Bellshaw @BellshawGeorge
Djokovic on heat: 'I prefer not talking about how difficult it is. We are blessed to play the sport we love. Just trying to cherish the moment of the court. I'm not 21 anymore, I still don't feel old but at same time there is a little biological clock that's not in your favour.'
Keys appears in good shape to match her 2017 campaign where she was a finalist in New York, and the 23-year-old put he foot on the accelerator as she beat Cibulkova, 6-1, 6-3.
Her dominance was clear for everyone to see as she pummelled her opponent, never giving her time to breathe in sweltering conditions.
The No. 14 seed had the victory wrapped up after one hour, 16 minutes, and she remains a credible threat to the field.
No. 20 seed Osaka won her opening set against Sabalenka, but the 20-year-old rising star from Belarus fought back to level the match.
However, it was Osaka who had the most left in the tank in a gruelling battle, and the Japanese talent booked a place in the last-eight—winning 6-3, 2-6, 6-4.
Nishikori made it a fine day for Japan as he proceeded to the next round, with Kohlschreiber failing to find form or rhythm.
The No. 20 seed raced to a two-set lead, and the German had no answer as Nishikori closed out the match.
The contest finished 6-3, 6-2, 7-5, and Nishikori could be a dark horse in the late stages of the tournament if he continues in the same fashion.
All stats via U.S. Open's official website.