Andy Murray admits he faces an uphill battle to recover from his five-set duel with France's Adrian Mannarino at the US Open.

Murray had to come from two sets down to beat the left-hander 5-7 4-6 6-1 6-3 6-1 and while the Scot's fightback was clinical, the match finishing in a modest three hours and 17 minutes, it was made all the more energy-sapping by the punishing heat.

Temperatures pushing 33 degrees Celsius at Flushing Meadows had already seen American Jack Sock retire mid-match against Ruben Bemelmans with heat exhaustion and Murray was also affected as he called for ice-towels at changeovers and hunched over after a number of lengthy points.

"I was bending over because I was out of breath," Murray said.

"It was extremely tough conditions, especially in the beginning of that fourth set, with some unbelievable points back-to-back with a lot of running. I was tired."

Murray showed his iron resilience again to reach his 30th consecutive grand slam third round but the repercussions of a lackadaisical opening may yet hurt the British number one's bid for a third major title.

He plays Brazilian Thomaz Bellucci in round three on Saturday, and should he beat the world number 30, a fiendish draw means Murray may have to go through Kevin Anderson, Stan Wawrinka and Roger Federer just to reach the final in New York.

The facts are that prior to this tournament, Murray had played 22 five-set matches at grand slams, but only once went on to reach the final after a five-setter in the first four rounds.

He has reached eight major finals so far and played 16 five-set matches in the fourth round or earlier.

"In these conditions, obviously it can take its toll later on in the tournament," Murray said.

"But unfortunately you don't always have the luxury of winning matches in straight sets. Sometimes you have to fight through if you're not playing your best.

"Thankfully the last three sets were fairly comfortable in terms of the scoreline, otherwise I could have been out there much longer and it would have been very tough to recover from.

"Normally the next day is when you feel it, when you wake up, you get stiff and sore.

"But that's why I obviously surround myself with the team that I do to recover as best as possible."

The sweltering conditions and Sock's withdrawal revived calls for the ATP to introduce a heat rule for men's competitions, as the WTA currently have for women's.

WTA regulations state female players can take a 10-minute break between the second and third set when the heat stress index - measured on temperature and humidity - exceeds 30.1 degrees Celsius, but there is no similar rule for men's matches.

"When it's extremely hot and humid, it helps to have that break," Murray said.

"I don't know exactly what it's for but I guess you get the chance to sort of go off and change, get under a cold shower if you want to."

Wawrinka, Murray's potential quarter-final opponent, believes it is odd the rules are not the same for both sexes.

"That's just a question I am always asking myself - why they have the heat rule if it's dangerous for them after two sets to have 10 minutes and why we don't have anything for five sets that's maybe there?" Wawrinka said.

"That is the question if we want to see something for the guys."