Former champions Wawrinka and Del Potro also into third round at Flushing Meadows
US Open: Nadal speeds through, Murray dumped out as heat extreme heat takes toll on Tsitsipas
World No 1 and defending champion Rafael Nadal steamed into the third round on another scorching day at the US Open on Wednesday with a straight-sets win over Vasek Pospisil.
Nadal, spared the worst of the heat thanks to his late-night start on Arthur Ashe Stadium, sped past Pospisil 6-3, 6-4, 6-2.
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."
Former champions Stan Wawrinka and Juan Martin del Potro also advanced, while 2012 winner Andy Murray, like Wawrinka back at Flushing Meadows after missing last year through injury, was bounced by Fernando Verdasco.
Wawrinka survived searing mid-day heat and a spirited Ugo Humbert.
"I knew it would be a difficult match," said Wawrinka, who was pleased to find himself feeling fit after prevailing 7-6, 4-6, 6-3, 7-5 in three hours and 21 minutes.
"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.
Meanwhile Murray, the 2012 US Open winner, who was playing his first grand slam in 14 months, is still fighting to find full fitness after hip surgery in January.
He gave himself a mixed review after a 7-5, 2-6, 6-4, 6-4 loss to Spain's Fernando Verdasco, whose only win over Murray in 14 prior meetings was at the 2009 Australian Open.
"I think some of the tennis I played today was some of the best I've played since I had the surgery or since I came back," Murray said.
He was disappointed to surrender the first set after holding a set point, but pleased he had enough in the tank to push Verdasco through five break points in the final game before the Spaniard converted his third match point.
Even with his current limitations, Murray is a dangerous opponent, Verdasco said.
"He's an unbelievable player, so talented. No matter how much his hip hurts or whatever, he's going to fight and he's going to put all the balls he can in and run," Verdasco said after earning a third-round meeting with 2009 champion Del Potro.
The third-seeded Argentine, who career was nearly ended by wrist injuries after his maiden grand slam triumph here, defeated American Denis Kudla 6-3, 6-1, 7-6.
Del Potro was delighted to get through in three sets, and not spend too long in the steamy sunshine.
Organisers 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, the Greek 20-year-old who came in with high expectations after beating four top 10 players en route to the Toronto Masters final, 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, who won his second ATP title of the season at Winston-Salem last week, booked a third-round meeting with 20th-seeded Croatian Borna Coric, a 7-6, 6-2, 6-3 winner over Spain's Roberto Carballes Baena.
Fifth-seeded South African Kevin Anderson, runner-up to Nadal last year and a runner-up to Novak Djokovic at Wimbledon, 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.