Hampered by a groin injury, the six-time champion appeared headed for a shock exit on Tuesday night against unseeded American Tennys Sandgren before rallying for a miraculous 6-3 2-6 2-6 7-6 (10-8) 6-3 victory.

It earned the 38-year-old 20-times major champion a record-extending 46th grand slam semi-final appearance.

Bidding for his first final four spot and to become the first US man in the Open semi-finals since Andy Roddick in 2009, Sandgren had seven match points in the fourth set.

But the moment got the better of the world No.100 while Federer - playing in his 1512th career match - held his nerve.

"I think I got incredibly lucky today," Federer said.

"I don't deserve this one but I'm standing here and I'm obviously very, very happy.

"I was just hoping that maybe he was not going to smash the winner, you know on that one point, that he would maybe keep the ball in play."

Federer needed a medical time-out in the third set, but didn't believe the injury would hinder his chances of a record-equalling seventh Open crown.

"I've got nothing to do tomorrow and then I play at night so you do feel better in a couple of days and then you just never know again," he said.

"With these lucky escapes, you might play without any expectations anymore because you know you should really be skiing in Switzerland.

"So there you go - I'm lucky to be here and I will make the most of it."

Federer looked in full control early, taking the first set in 34 minutes, but Sandgren broke early in the second set and as the wheels started to fall off a hush fell over the venue.

Down 0-3 in the third set, the world No.3 was angered when he received a code violation warning for a verbal obscenity after being reported by a lineswoman.

He then called for a physio and left the court for treatment.

Federer, who has never retired mid-match, said he was determined to see it out.

"I believe in miracles - it could be rain, there could be stuff, I don't know - it wasn't bad enough that I thought I was going to get worse, it was just stiff and tight.

"I thought I would just let him finish me off in style - and he didn't."

Sandgren had three match points on Federer's serve at 5-4 but was unable to find the winner with the set then going to a tiebreak.

It was locked at 3-3 when a ballkid bizarrely ran into Sandgren's leg at the changeover, with the American taking time to walk off the blow.

Unrattled, he steamed ahead to lead 6-3 but again couldn't close it out.

He had one final chance at 8-7 but Federer showed his poise to level and then clinch the set.

A deflated Sandgren was unable to lift in the fifth, with the 20-time major champion breaking to go ahead 4-2 before sealing victory.

Federer will face Novak Djokovic for the 50th time in a mouth-watering semi-final after the defending champion outlcassed Milos Raonic 6-4 6-3 7-6 (7-1) on Tuesday night.

"Draws are not getting easier," Federer said.

"I'll have to play better than I did today otherwise I really am going skiing."

Australian Associated Press