World No.1 Rafael Nadal has withdrawn from the ATP World Tour Finals with a knee injury after his first-up defeat to David Goffin.

Nadal pulled out of last week’s Paris Masters and was clearly still below 100 per cent in his 7-6 (7-5) 6-7 (4-7) 6-4 loss to Belgian’s Davis Cup hero on Monday night.

“My season’s over,” the Spaniard said.

“I had a commitment to the tournament, to the city, to myself.

“My commitment was to try … I’ve missed this event too many times in my career.”

The reigning French and US Open champion said he knew midway through the match his tournament was over — win or lose.

“I’m not ready to play,” Nadal said.

“I fought a lot knowing it was my last match of the season.”

Nadal gallantly saved four match points to force a third set but not a fifth, with Goffin not to be denied the biggest scalp of his career.

Nadal’s shock exit has blown the season-ending showpiece wide open and came after Grigor Dimitrov beat Dominic Thiem 6-3 5-7 7-5 in an equally thrilling opening clash in the Pete Sampras Group at London’s O2 arena.

Goffin and Dimitrov will clash on Wednesday, with the winner certain to secure a semi-final spot.

Thiem will play Nadal’s replacement, fellow Spaniard and first alternate Pablo Carrena Busta, needing to win to keep his title hopes alive.