Olympic champion Murray reaches last 16 at US Open

British third seed Murray will next face the winner of a later match between American James Blake and 15th-seeded Canadian Milos Raonic.

Murray, four times a Grand Slam runner-up but still seeking his first major title, had lost only one set in six prior matches against Lopez but battled in heat and humidity to avoid being ousted, trailing in all three tie-breakers.

"It was a very tough match," Murray said. "Three tie-breakers, could have gone either way. He was up in all three of them. It was hot and humid. We played some good tennis, especially toward the ends of sets."

Murray's 250th career match victory on hardcourts, what he says is his favorite surface, came after three hours and 53 minutes when Murray won the final four points, the last of them on a netted Lopez backhand.

"This was the first match I had played in this humidity for a while so it was tough," Murray said. "I would like to be in slightly better form but these matches have helped with that."

With coach Ivan Lendl watching in stoic fashion, Murray won his 10th match in a row over a southpaw, having not lost to a lefty since Rafael Nadal beat him in last year's US Open semi-finals.

Murray took the first set but nearly faced a set point in the 10th game of the second set. Lopez raised his arm to ask for a replay-system review of a line call that would have, if successful, given him a crucial break chance.

But umpire Jake Garner denied the request, saying it had come too late even though it clearly had not, saying he had watched Lopez but seen no signal after his shot had been called out.

"You didn't watch me at all. You didn't see," Lopez fumed. "It was so clear. I challenged immediately. It's a very important point. You will see on TV."

Murray held and they battled into a tie-breaker in which Murray fell behind 4-1 only to capture six of the last seven points, including the final four to seize command of the match.

At 6-5, a Lopez cross-court backhand was called in but Murray asked for a replay and as he waited for it yelled to Garner, "Neither of you called that." Garner replied, "Yeah, I know. It's wide."

The replay system agreed moments later and Lopez miss-hit on the next point to hand Murray the set.

After exchanging breaks twice in the third set, Murray netted a forehand to give Lopez a 5-4 lead and the Spaniard held to force a fourth.

Murray saved a break point in the fifth game of the fourth set and Lopez rescued two in the eighth.

In the 11th game, Lopez had a break-point chance and a 23-shot rally ensued that saw the Spaniard roam all across his side of the court before finally netting a forehand, the crowd standing and cheering in appreciation.

In the tie-breaker, Lopez seized a 4-3 edge on a service winner but netted a backhand and sent a forehand wide before Murray's backhand winner set up match point.