Andy Murray blamed the sweltering conditions for his hard-fought four-set victory against 30th-seeded Spanish left-hander Feliciano Lopez.
Murray, trying to add his first Grand Slam title to the Olympic gold medal he captured last month, won eight fewer points in the match but was tougher when the stakes were highest.
The third seed has a great record against Lopez but this match was tight from the start and Murray looked in trouble in the fourth set as he began to struggle in the heat.
But he hung tough and eventually edged his third tie-break of the match to come through 7-6 (7/5) 7-6 (7/5) 4-6 7-6 (7/4) to win his 250th career match on hard courts and set up a last-16 clash with the big serving Milos Raonic.
The Scot said: "It was a very, very tough match. I just had to keep fighting. It was very hot and humid in the middle part of the match and I was struggling a little bit with that. But I thought we played some good tennis, especially at the end of the sets."
Murray suffered with cramp in his first-round win over Alex Bogomolov despite it being straight sets, and he said: "Physically I'd like to be in slightly better form but these matches help with that.
"Normally after Wimbledon I have three weeks in Miami to help with this but I didn't get that because of the Olympics. But it's a nice problem to have, that's for sure.
"It's the first couple of matches I've played in this heat and humidity for a while so it's been tough."