The 19-time Grand Slam champion romped the opening set of the ATP finals semi-final 6-2 and looked on course for a routine win.

But the unfancied Goffin powered back to take the second 6-3 before breaking early in the third. He went on to wrap up a shock three-set win.

Speaking about why Roger Federer slipped up. Rusedski said: "I don't think it's anything physical.

"It's just concentration. He needs to focus, get the footwork right and do the right things out there.

"If he does that he should get through to the final.

"But we're just surprised whenever Federer loses a set that we don't think he's supposed to.

"He's a human after all."

Federer was bidding to win his seventh ATP Finals title in London but Rusedski's prediction didn't come up trumps.

Goffin held his serve - and his nerve - to record a 3-6 6-3 6-4 win.

He will take on Grigor Dimitrov or Jack Sock, who meet tonight, in Sunday's final.