The curtain was 90 per cent of the way drawn on Roger Federer's Australian Open campaign for 2020.

Momentum was completely against the Swiss master as American Tennys Sandgren applied the pressure as he raced away to a two sets to one lead.

In what was a drama-filled third set, the frustrations got the better of Federer who blew up after missing a routine backhand down the line.

While not entirely clear from the on-court microphones what his actual spray was, one word in particular was evident. A nice and loud F-bomb from the normally calm legend rang out across Rod Laver Arena and earned him a verbal obscenity violation.

Things then went from bad to worse as Federer called for the trainer before taking a medical time-out. His return to the court left commentator Jim Courier to proclaim things weren't right.

"Houston we have a major problem here with Roger Federer," Courier said.

"He is in major trouble here, physically. Not exactly sure if it's back, groin, hip, whatever the treatment is, it's real. He just doesn't have it."

Somehow after looking like he simply couldn't move freely around the court, Federer's body sprung back to life and he slowly began to wrestle back the momentum.

An arm-wrestle fourth set ensued as Federer saved not one, not two, but seven match points and pushed the contest into a deciding fifth set.

From there it was clear the tennis legend held all the momentum as Sandgren looked powerless to stop the inevitable.

"Gotta get lucky sometimes. I think I got incredibly lucky today … as the match went on I started to feel better again and all the pressure went away," Federer said post-match.

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

"I believe in miracles."

The legend was hampered by injuries, but stated he isn't a fan of calling for the trainer in the middle of games.

"I don't like calling the trainer because it's a sign of weakness and whatever.

I believe in miracles. There could be rain, there could be stuff. It wasn't bad enough where I thought it'd get worse. It was just stiff and tight.

"I just thought I'd let him finish me off in style. "I'm lucky to be here so I'm going to make the most of it."

The miraculous performance puts him through to another semi-final and left the tennis world in absolute awe of what he had just pulled off.

The stunning comeback victory puts Federer into the semi-finals where he'll take on the winner of Novak Djokovic and Milos Raonic.