Spain's Rafael Nadal gestures to fans after losing his group stage match against Belgium's David Goffin Action during the 2017 ATP Finals at the O2 Arena, London, Britain, Nov 13, 2017. [Photo/Agencies]

LONDON - World No 1 Rafael Nadal withdrew from the 2017 ATP Finals after suffering a surprise defeat to seventh seed David Goffin of Belgium in his first group match at the O2 Arena here on Monday.

The 16-time Grand Slam winner, who has secured the top place in the year-ending ATP Rankings, was apparently struggling but showed inspiring fighting spirit, saving four match points in the second set before losing 7-6 (5), 6-7 (4), 6-4.

Goffin, playing the ATP Finals for the second time, took command in the third set, serving for the match with a 14th ace of the match.

"I am really happy. It was such a good atmosphere tonight," Goffin said.

Nadal, who pulled out of he Paris Masters at the quarterfinals 10 days ago, had admitted before the match that his knee was "not perfect". His countryman Pablo Carreno Busta will take his place in Group Pete Sampras.

Nadal has won 75 tour-level titles, including 30 ATP World Tour Masters 1000 crowns, but he has never won the ATP Finals. He qualified for the event 13 times but competed only in eight editions.

"My season is finished," he said after the match. "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 tried, but [it] seriously was a miracle to be very close in the score during the match."

In the day's other match, Bulgaria's Grigor Dimitrov upset fourth seed Dominic Thiem of Austria in a topsy-turvy match 6-3, 5-7, 7-5.

The sixth seed broke Thiem three times, including late in the third set as Thiem struggled to hold his nerves.

"It's not only big for me, I think it's for my whole country, for Bulgaria," said Dimitrov, who was the first player from his country to make the ATP Finals in its 48-year history.

"It's great for me to show that everything is possible. You push the boundaries, doesn't matter where you're from, it's all up to you, everything is in your hands. Basically whatever you put in is what you get."