LONDON (Reuters) - Austria’s Dominic Thiem produced a fearless display of attacking tennis to beat Rafa Nadal 7-6(7) 7-6(4) in a high-quality duel to book his place in the last four at the ATP Finals on Tuesday.The world number three struck 37 clean winners to claim his second round-robin victory and reigning champion Stefanos Tsitsipas’s narrow defeat of Andrey Rublev later guaranteed the Austrian a semi-final spot with a match to spare. “Definitely one of the better matches I have played so far in my career,” this year’s U.S. Open champion Thiem said. Crucially he saved a couple of set points in the first tiebreak 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.