Gael Monfils let slip two match points which would have given him a famous US Open triumph over Roger Federer on Thursday and said: “It’s cool”.

The 28-year-old went down 4-6, 3-6, 6-4, 7-5, 6-2 to the 17-time Grand Slam champion in the quarter-finals after seeing two match points in the 10th game of the fourth set as Federer reached his ninth US Open semi-final to stay on course for a sixth title.

The 33-year-old Swiss second seed, bidding to become the oldest winner of a Grand Slam title in more than 40 years, was staring down the barrel of a demoralising exit when French 20th seed Monfils had him on the ropes.

But once he had averted the danger, Federer then swept past the exhausted Monfils in the final set.

“I think it was cool. I think he hit two big serves and a good forehand volley and then a good forehand down the line. I think, you know, I did my best, so it was OK,” said Monfils, who had been bidding to make just his second semi-final at a major after a run to the French Open last four in 2008.

“Then he played good. He had the set. Then physically I had a drop, five minutes. It came quick. He started to be very offensive. So then it was very tough to handle.”

Monfils had roared into a two-sets lead, firing winners off both flanks as Federer tried and failed to convert break points.

He even berated umpire Carlos Ramos for his officiating telling him that he didn’t know what was wrong with him before an uncharacteristic bout of petulance saw him chop his racquet into the net.

Monfils admitted that the Swiss was able to adapt to the impending crisis, a tactic which has served him well down the years of his storied career.

“That’s why he’s Roger Federer, because he change so many times. He starts with chipping very low. I think I handled it good. So then he stick with longer points. It was 50-50, and then he try to come to the net very often,” said Monfils when asked to analyse the tactics.

“It was a bit better for him. Then suddenly he started to mix everything. That’s why he’s the greatest player, because he can do everything.”