Rafael Nadal beat Vasek Pospisil 6-3, 6-4, 6-2 and reached the third round at the US Open.
His only hiccup came in the second set, when he dropped his serve to fall 2-4 down.
"The conditions are tough," Nadal said. "It was important to start very well with that break. To win in straight sets is always very positive, especially in these conditions. It's almost midnight, very happy about the victory."
Stan Wawrinka survived searing mid-day heat and a spirited Ugo Humbert. The Swiss won 7-6 (7/5), 4-6, 6-3, 7-5.
"I knew it would be a difficult match," said Wawrinka.
"My level is there. I'm playing really good tennis," added Wawrinka, who was unable to defend his 2016 title after undergoing two knee surgeries last year.
"I think there is a good chance that I'm playing better in the next round," said the Swiss, who will face Canadian Milos Raonic for a place in the last 16.
Juan Martin del Potro defeated American Denis Kudla 6-3, 6-1, 7-6 (7/4).
Del Potro was delighted to get through in three sets, and not spend too long in the steamy sunshine.
Organizers again implemented an extreme heat policy that allowed both men and women a 10-minute heat break in matches extended beyond straight sets.
Stefanos Tsitsipas said the heat took a mental as well as physical toll.
"You don't have fresh air to breathe -- you breathe this heat that's coming, the moisture that's in the air. So you feel like you are empty," said the 15th-seed, who fell 6-4, 6-3, 4-6, 6-3 to Russian Daniil Medvedev.
Medvedev booked a third-round meeting with 20th-seeded Croatian Borna Coric, a 7-6 (7/4), 6-2, 6-3 winner over Spain's Roberto Carballes Baena.
Kevin Anderson made quick work of France's Jeremy Chardy, 6-2, 6-4, 6-4.
Anderson next faces rising Canadian star Denis Shapovalov, who labored to a 6-4, 4-6, 5-7, 7-6 (7/2), 6-4 victory over Italian Andreas Seppi.
