Murray shows his durability to turn up the heat on Lopez

Scot books place in second week of US Open after gruelling encounter played in intense humidity

Andy Murray grimaces in the heat of the Louis Armstrong Stadium on his way to a four set victory over Feliciano Lopez (AP)

Andy Murray has a reputation as one of the most durable players in the game but the 25-year-old Scot was stretched to his limits here last night before booking his place in the second week of the US Open. The gruelling heat and humidity on another testing day in New York took their toll on Murray, who nevertheless held on to beat Feliciano Lopez 7-6 7-6 4-6 7-6 after three hours and 53 minutes.

Murray handled the conditions well for the first two sets, in which he was happy to bide his time and wear down Lopez with his consistent rallying, but thereafter the world No 4 wilted before gritting his teeth in the fourth set to secure victory. In the fourth round he will play the winner of last night's later match between Milos Raonic and James Blake.

"I just had to keep fighting to the end," a relieved Murray said afterwards. "It was very hot and humid in the middle of the match and I was struggling with it.

"Normally I like to have three weeks in Miami after Wimbledon to get accustomed to the conditions here and I didn't get that because of the Olympics. It was a nice problem to have, though, that's for sure. This is the first couple of matches I've played in humidity like this for a while now, so it's tough."

After dropping only 13 games in his first two matches Murray was made to toil by Lopez, a big-serving Spaniard who had won only one set in his previous six meetings with the Scot. Ultimately the difference between the two men was Murray's rock-solid consistency in the three tie-breaks, each of which he contested with steely resolve.

Having played his first two matches in the cavernous Arthur Ashe Stadium, Murray had his first experience this year of the 10,000-capacity Louis Armstrong Stadium, which used to be the main show court. The temperature was already up to 32C by the time the players came on court shortly after 1pm and they were soon draping ice towels around their necks during the changeovers.

The match, ironically, began as a slow-burner, with only eight points won against serve in the first nine games. With Murray leading 5-4, however, both men caught fire. Lopez went 0-40 down but promptly dug himself out of trouble with some big serves. Murray followed suit in the next game, recovering from 15-40 down with an ace and then a bold foray into the net. Having served two double faults in the game, he kept telling himself to "cool it".

The tie-break went with serve until 5-5, when Lopez hit a wild forehand beyond the baseline. Murray, sensing his moment, charged forward, forcing Lopez into another backhand error to take the set after 59 minutes.

When Murray broke in the opening game of the second set it seemed likely that he would quickly draw clear. The Scot, however, is nothing if not unpredictable and from 3-2 up he suffered an alarming lapse, losing eight points in a row and handing back his break of serve.

The set went into another tie-break, in which Murray's composure and Lopez's unreliability were once again to prove decisive. From 5-3 up Lopez went into meltdown. The world No 31 struck a wild forehand wide, put an easy backhand long, failed to return a serve and finally shanked a forehand.

For all Murray's many qualities one of his biggest weaknesses in recent times has been his failure to hold serve immediately after a break. In the third set he did that twice. The Scot was suddenly looking tired, which appeared to give Lopez a new lease of life. When he broke to lead 5-4 the Spaniard leapt in the air and went on to serve out for the set.

Normally Murray can be relied upon to chase down every ball, but by the fourth set he was conserving his energy. Both men failed to take break points as the set went into another tie-break. Once again it was tight, but when Lopez served at 4-5 Murray hit a sensational backhand cross-court pass to create match point. Lopez, predictably enough, handed the Scot victory when he put an attempted drop shot in the net on the following point.

Murray said he had struggled both with the tough conditions and with Lopez's game. "When you play someone with a big serve like Feli's it's mentally tough because you can have games when you're just not touching the ball," he said.

Marin Cilic, who beat Murray in the fourth round here three years ago and is a possible quarter-final opponent this week, beat Japan's Kei Nishikori 6-3 6-4 6-7 6-3. In the fourth round Cilic will play Martin Klizan, who followed up his victory over Jo-Wilfried Tsonga by beating another Frenchman, Jeremy Chardy, 6-4 6-4 6-4. It was Klizan who lost the deciding rubber of Slovakia's Davis Cup tie against Britain in a shopping centre in Glasgow but he has risen to be No 52 in the world.

Roger Federer reached the last 16 with a third consecutive straight-sets victory, beating Spain's Fernando Verdasco 6-3, 6-4 6-4. Jack Sock's attempt to become only the second American teenager to reach the fourth round in the last 21 years ended when he ran out of steam against Spain's Nicolas Almagro, losing 7-6 6-7 7-6 6-1.