Dominic Thiem was able to celebrate a place in the last four of the Nitto ATP Finals after a stunning win over Rafael Nadal that he rated as one of the best of his career.

At the end of a brilliant contest full of intensity and explosive shot-making, it was US Open champion Thiem who emerged a 7-6 (7) 7-6 (4) winner.

He was left waiting on the result of the evening match to see whether he would have more work to do but defending champion Stefanos Tsitsipas' 6-1 4-6 7-6 (6) victory over Andrey Rublev secured his passage.

Nadal and Tsitsipas will battle it out on Thursday for the second qualifying spot, while Rublev's chances of progressing on his debut are now over.

Thiem was thrilled with his performance, saying: "It's definitely one of the better matches I've played so far in my career. I think it came close to the second group match of last year against Nole (Novak Djokovic). I think we both played great tennis. Very, very small things make the difference.

"The second set I think had everything. I played probably the best game of the match to make the rebreak and then another good tie-break. It was a great match which I will remember for a long time.

"I think that today I played a little bit higher level than at the US Open. It was maybe the best match from me since the restart of the tour and that makes me super happy."

Despite losing the match, Nadal was left with positive feelings about his chances of winning a first ATP Finals title.

"It has been a great match of tennis decided for a few small details," he said. "Just well done to him. He played I think an amazing match, and I played well too.

"My feeling is not negative. I lost but I had plenty of chances in the first and then I had a break in the second. I think my chances are bigger to have a very good result now than five days ago because the level of tennis, even if I lost today, is much higher."

The match was one of the highest-level contests seen in the tournament's 12-year stay at The O2 and the only shame was that there were no fans to witness it.

Nadal and Thiem had met on 14 previous occasions, all but two of them on clay. The two matches on hard courts were both corkers – a US Open quarter-final in 2018 that Nadal won on a deciding tie-break and a clash at the same stage of the Australian Open earlier this year won by Thiem.

The Austrian won all three of his sets that day on tie-breaks, and the first set here went the same way after 12 pulsating games in which the pair were so well matched that they won exactly the same number of points.

It appeared Nadal would come out on top this time when he led 5-2 with two serves to come but Thiem won both of those points and then, after the Austrian double-faulted, Nadal dumped a snatched backhand into the net on set point.

At 34, the nerves have begun to penetrate Nadal's previously ice-cold veins at the biggest moments, and that was evident again here.

Thiem, who won a long-awaited first grand slam title in New York, now arguably carries the mantle of bravest when the stakes are highest and he saved another set point with a thumping forehand winner before staging a repeat to steal the set.

There was no let-up in quality in the second set, with Thiem responding straight away after Nadal secured the first break of the match in the seventh game.

This time Thiem took hold of the tie-break and did not let go, once again applying full throttle and reaping considerable rewards.

Tsitsipas looked like he would have a straightforward evening's work when he wrapped up the first set against Rublev in just 19 minutes.

But the 23-year-old Russian dug deep to turn things around and recovered from 2-5 in the deciding tie-break only to double fault on his only match point.

Tsitsipas, who lost his opening match to Thiem, said: "I'm very happy that I showed determination and a willingness to not give up when he had match point. I produced some really good tennis and that helped me take the win."

Britain's Joe Salisbury and American Rajeev Ram must win their last group game to reach the semi-finals of the doubles after losing 7-6 (5) 6-0 to Wesley Koolhof and Nikola Mektic.