Nadal pulled out of last week’s Paris Masters and was clearly still below 100 per cent in his three set loss to Belgiums’ Davis Cup hero.
“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 Thursday, 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.