The world No 1 was stunned 7-6 (7-5) 6-7 (4-7) 6-4 by David Goffin in his opening match at the season-ending event on Monday as he sought to win the trophy for the first time.

It is the sixth time Nadal has withdrawn from this competition in 13 years and he will now be replaced by fellow Spaniard Pablo Carreno Busta.

The Spaniard came into the tournament with doubts surrounding his fitness after pulling out of the Paris Masters through injury, but initially showed no obvious ill-effects during the round-robin encounter with Goffin.

But by the end, the knee problem that had threatened his participation in London this week was clearly causing him pain and he confirmed his withdrawal immediately after the match.

I have to accept the bad news, bad feelings. It's the end of my season, but it's been an unforgettable one

Rafael Nadal

Nadal said during a post-match press conference: "My season is finished. Yeah, I had the commitment with the event, with the city, with myself. I tried hard. I did the thing that I had to do to try to be ready to play. But I am really not ready to play.

"I cannot hold with enough power to keep playing. I tried, but seriously it was a miracle to be very close in the score during the match. It really doesn't make sense."

He later told Sky Sports' Mark Petchey: "Yeah, of course it's been a tough day. But I knew before the tournament that it was going to be a tough situation. I wanted to try, but it was not possible. I have to accept the bad news, bad feelings. It's the end of my season, but it's been an unforgettable one."

He was due to play Grigor Dimitrov and Dominic Thiem in the remainder of the round-robin format before the knockout stages begin on Saturday.