Cilic, just five days younger than del Potro, had stunned World No. 2 Andy Murray in the fourth round to reach his first Grand Slam quarter-final and carried his blistering form into the first set of his clash with del Potro – breaking the Argentine in the fifth game before going on to close out a 6-4 one-set lead.

A poor service game from del Potro, including two double faults, handed Cilic an early break of serve in the second set and the Argentine’s hopes in the match appeared to hang in the balance as Cilic moved to within two points of a double break lead at 1-3, 0/30 on del Potro’s serve.

It proved to be the turning point in the match as del Potro rose to the occasion and went on to win 11 of the next 13 games and take a two-set-to-one lead. Where Cilic’s forehand had been potent in the first set and a half, it proved unreliable as del Potro mounted his comeback and fell down against the Argentine’s improved aggression from the back of the court as he turned the match around.

‘Definitely he started to play much better and not giving me so many chances to dictate the game,” Cilic said about the turnaround. “He was not missing and also putting a lot of pressure on me, and then it was tough for me to hold that level of game all the time.”

Del Potro raced to a 5-0 lead in the fourth set against an increasingly despondent Cilic, who had no answer to the Argentine’s dazzling play. The Croatian restored some pride and avoided a whitewash as he broke del Potro in the sixth game as he served for the match. But his reprieve was short lived and another forehand error brought up match point for the Argentine on Cilic’s serve at 1-5, 30/40. As another Cilic forehand found the net – one of 53 unforced errors he committed in the match – del Potro clinched victory after two hours and 33 minutes of play in extremely blustery conditions on Arthur Ashe stadium.

World No. 17 Cilic also lost in four sets to del Potro in the Australian Open fourth round, having won the first set on that occasion too. He was bidding to become the first Croatian man to reach the US Open semi-finals since Goran Ivanisevic in 1996, and the fourth overall to reach the semi-finals at a major.

The Zagreb resident is expected to return to the Top 15 in the South African Airways 2009 ATP Rankings following his result at the US Open. He reached a career-high No. 13 on 18 May this year following two ATP World Tour titles at the start of the season in Chennai (d Devvarman) and Zagreb (d. Ancic).

“I’m feeling unbelievably happy that I got to this stage,” he said. “Before the tournament I wasn’t in great situation with losing two first rounds and one second round in Cincinnati.

“I had good experience here, some great matches, one comeback after being two sets to love down, and beating Murray also. I think putting my game on another level now. Of course I have little more self belief that I can do well and that I’m also playing good tennis.”