"My level is there, I'm playing really good tennis," added the Swiss, who will face Canada's Milos Raonic for a place in the last 16. "I think there's a good chance he's playing better in the next round. "
Meanwhile, Murray, the 2012 US Open winner who played his first Grand Slam in 14 months, is still struggling to find a good shape after hip surgery in January.
A mixed critique was made after a 5-7, 6-2, 4-6, 4-6 loss to Spaniard Fernando Verdasco, whose only win over Murray in 14 previous meetings was at the 2009 Australian Open.
"I think some of the tennis I played today were some of the best I've played since I had the surgery or since I came back," said Murray, who bothered to deliver the first set after maintaining a set point, but satisfied enough. in the tank to push Verdasco through five break points in the final game before the Spaniard converted his third match point.
"There were too many ups and downs for my taste," said Murray.
Verdasco certainly was not surprised by the "ups" and said that even a limited Murray was still dangerous.
"He's an incredible player, very talented, you know that no matter how much his hip hurts or whatever, he's going to fight and he's going to put as many balls as he can and run," Verdasco said after winning a third-round meeting with another former champion, the third seed Juan Martin del Potro.
The Argentine, whose career almost ended with wrist injuries after his triumph at the 2009 US Open, defeated American Denis Kudla 6-3, 6-1, 7-6 (7/4).
Del Potro was delighted to pass in three sets, and not spend too much time under the blazing sun.
Murray said the shadows cast in the huge Arthur Ashe stadium provided a slight respite from the suffocating temperatures.
The organizers again implemented a policy of extreme heat that allowed men and women a 10-minute break in extended matches beyond straight sets.
World number one and defending champion Rafael Nadal was saved from the worst of conditions, although he was still embarrassed when he crowned Ashe Stadium's action against Canadian Vasek Pospisil.
Stefanos Tsitsipas, the 20-year-old Greek who entered with great expectations after beating four players in the top 10 en route to the Toronto Masters final, said the heat had a physical and mental effect.
Breathing the heat
"You do not have fresh air to breathe, you breathe this coming heat, the humidity that is in the air, so you feel you are empty," said the 15th seed, who was undone by 64 unforced errors in a 6-4, 6 -3, 4-6, 6-3 defeat against Russian Daniil Medvedev.
Medvedev, who won his second ATP title of the season in Winston-Salem last week, booked a third-round match with the No. 20 seed, Croatian Borna Coric, winner of 7-6 (7/4), 6-2 , 6-3. about the Spanish Roberto Carballes Baena.
South African fifth seed Kevin Anderson, runner-up to Nadal last year and second to Novak Djokovic at Wimbledon, did a quick job with Frenchman Jeremy Chardy, 6-2, 6-4, 6-4.
Anderson will face emerging Canadian star Denis Shapovalov, who won 6-4, 4-6, 5-7, 7-6 (7/2), 6-4 over Italian Andreas Seppi.
The Austrian Dominic Thiem, ninth seed and finalist of the French Open, whose preparation at the US Open was affected by injuries and illness, advanced with a victory of 6-7 (5/7), 6-3, 5- 7, 6-4, 6-1. about the American Steve Johnson.
He will face another American, Taylor Fritz, who advanced 6-3, 3-6, 6-3 when Australian opponent Jason Kubler withdrew.