Dominic Thiem serves against Rafael Nadal on Tuesday in London, England. Photo: VCG

Austria's Dominic Thiem produced a fearless display of attacking tennis to beat ­Rafael Nadal 7-6 (9/7), 7-6 (7/4) in a high-quality duel to book his place in the last four at the ATP Finals on Tuesday.

The world No.3 struck 37 clean winners to claim his second round-robin victory and reigning champion Stefanos Tsitsipas' narrow defeat of Andrey Rublev later guaranteed the Austrian a ­semifinal spot with a match to spare.

"Definitely one of the better matches I have played so far in my career," this year's US Open champion Thiem said.

Crucially he saved a couple of set points in the first ­tibreaker before whipping away a forehand to take an opening set in which there was barely a cigarette paper between the two.

Spaniard Nadal, bidding to win the only big title to elude him, engineered the first break of the match at 3-3 in the second set only for Thiem to reply in stunning fashion.

Nadal was on the brink of defeat when he slipped 0-40 behind serving at 4-5 after a wayward smash, but the 20-time Grand Slam champion dug himself out of a hole, saving three match points, one with a deft drop volley that clipped the tape.

Thiem was unshakeable, however, and played the sort of courageous tennis that has become his trademark as he forged his way into a 6-3 lead in the day's second tiebreak.

Nadal clawed one point back, but Thiem finished it off when his opponent dragged a backhand wide, a rare unforced error in an absorbing contest sadly played out in an empty O2 Arena.

Nadal said "small details" decided a superb contest and remains confident he can still win the elusive title, although on Thursday he faces a shootout with Tsitsipas, who survived an ambush by Russian debutant Rublev to win 6-1, 4-6, 7-6 (8/6).

"It's going to be another tough one but playing like this I am confident that I can have my chances in the tournament," Nadal, twice a runner-­up in London, said.

The quality of the rallies in the first set would have graced any final and heading into the tiebreaker both players had won 34 points, with hardly a free gift in sight.

So it was a surprise to see a couple of errors from both men in the breaker. Nadal served a double fault and Thiem, after battling back from 2-5 to 5-5, then threw in one of his own. Nadal netted a routine backhand on his first set point but was powerless as Thiem saved another with a rasping forehand.