The Swiss number two seed was staring at defeat in the fourth set in Arthur Ashe Stadium, facing two match points against his French opponent.

But a long Monfils backhand and a sublime Federer forehand took care of both of those.

Monfils soon faded as Federer took the driving seat to reach the last four, settling the match 4-6, 3-6, 6-4 7-5, 6-2.

"I still thought the finish line was far away for Gaël," said Federer of dropping the first two sets.

"I knew I could play better tennis.

"When I was down two match points, that's when I wasn't feeling too good any more.

"I thought, 'This is it, don't let him have an easy shot, and go down fighting.'

"But I served well and managed to turn it around."

Gaël Monfils was tantalisingly close to the US Open semi-finals before being beaten by Roger Federer ©AFP/Getty Images



He added: "I'm not sure I have ever saved match point before in a Slam.

"If that hasn't happened, I'm unbelievably happy that it was today."

Monfils more than played his part in a thrilling contest in front of a crowd of 22,000.

The 20th seed has reached the Grand Slam of a quarter-final four times before, most recently at this year's French Open.

His best Grand Slam showing was at Roland Garros in 2008, when he made it into the last four.

Federer will now play Croatia's Marin Cilic, who made light work of Czech Tomas Berdych.

The number 14 seed won in straight sets, 6-2, 6-4, 7-6.

Tomorrow is women's semi-final day at Flushing Meadows.

Shuai Peng of China will play Denmark's Caroline Wozniacki, before top seed Serena Williams of the United States takes on Russian Ekaterina Makarova.