New York: Roger Federer saved two match points to defeat Gael Monfils 4-6, 3-6, 6-4, 7-5, 6-2 and reach his ninth US Open semi-final Thursday, keeping alive his dream of 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 two match points in the 10th game of the fourth set. But once they were saved, the lifeblood was suddenly sucked out of the flamboyant but unpredictable 28-year-old Frenchman whose notoriously fickle stamina failed him in a one-sided fifth set.