Andy Murray reaches US Open semi-finals

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World number four Andy Murray turned back the challenge of big-serving American John Isner 7-5, 6-4, 3-6, 7-6 (7/2) on Friday to reach the semi-finals of the US Open.

The 24-year-old Scot withstood 17 aces from Isner to reach his fourth Grand Slam semi-final of the season.

"It's so frustrating playing against him because you feel like you're playing good tennis and it's so tough to break him," said Murray, who fired 14 aces himself.

Murray had fended off two break points in the ninth game of the fourth set to get to the tiebreak.

After an Isner double fault staked him to a 2-1 lead, three winners put Murray in control and he sealed the victory on his first match point when Isner's service return sailed wide.

It was an emphatic end to a match in which each break of serve proved precious.

Isner presented Murray with his first break chance with a double-fault at 5-5 of the opening set.

Murray seized the opportunity to pocket the set, and when he broke Isner to open the second set the momentum was all his.

Murray, playing in the US Open quarter-finals for the first time since his runner-up finish to Roger Federer in 2008, made the break hold up, gaining the initiative in the 10th game with a mighty backhand up the line and taking a two-set lead when Isner sent a backhand wide.

Isner, playing his first Grand Slam quarter-final, seized the initiative in the third set with a break in the second game.

Murray saved two break points, the second with an ace, and when Isner gave himself another chance Murray snatched his hat off in frustration.

The lone break proved enough for Isner, who opened the ninth game with three unreturnable serves to give himself three set points, and after Murray saved one closed it out with an ace.

"He served so well and put a lot of pressure on me," Murray said. "I'm relieved to win that fourth-set breaker."

In Saturday's semi-finals, Murray will face either defending champion Rafael Nadal or former champion Andy Roddick, who were playing later Friday.