LONDON - David Goffin produced a dazzling comeback to beat hot favourite and six-time champion Roger Federer 2-6, 6-3, 6-4 on Saturday and reach the final of the ATP Finals . Federer was quickly into his stride in front of a packed house at London's O2 Arena, winning the first set with ease and the chances of a comeback from the Belgian appeared remote.

But the seventh seed hit back early in the second set to break the world number two and, suddenly looking rejuvenated, went on to take level the match before breaking again early in the decider, with Federer unable to find the inspiration to come back.

"I have no words. I cannot describe how I'm feeling," said an emotional Goffin . "So much joy, so much happiness. I'm so happy to play the match I played, it was such a special moment. Honestly, I don't know (what worked). I think it was just the feeling of the day. I was a little bit nervous when I started the match but I started to feel the ball really well. The serve was there right to the end. It was my day."

Goffin , 26, engineered a remarkable turnaround against an opponent he had never beaten in six previous encounters. At the start of the match he had no answer to Federer 's variety and weight of shot, lacking the weapons to put the Swiss under sustained pressure and Federer looked on course for an easy win.

Unveiling his full array of shots in front of a purring crowd at London's O2 Arena, including a remarkable low backhand volley at the net, he was briefly held up by Goffin but sealed the set 6-2 with an ace.

But in a surprising twist Goffin quickly found himself 3-0 up in the second set after breaking Federer at his first opportunity and suddenly looked a different player, more aggressive and unwilling to allow Federer to dictate play as the Swiss player lost his precision.

Federer , 36, appeared more focused at the start of the third set but Goffin broke in the third game to take a 2-1 lead and heap the pressure on the Swiss. In a tense fourth game Federer earned a break point but Goffin fended off his charge to hold and, looking increasingly assured, held again for 4-2.

Goffin rose to serve for the match at 5-4 and made no mistake, holding his nerve to secure a famous victory.

A disappointed Federer said Goffin , who will play either Grigor Dimitrov or Jack Sock in Sunday's final , was the better player.

"It's a bit disappointing for me because it's indoors, it's a court I like to play on," he added. "But I had my chances and missed them," he said. "When he had them, he was very committed. I think that was the difference today."