Dominic Thiem produced a unique brand of tennis to put a major dent in Novak Djokovic's hopes of closing the gap on Rafael Nadal in the race for year-end world No 1 on Tuesday.
Thiem ripped 50 winners, playing arguably some of the finest tennis of his career, to come out on top of an exhilarating contest 6-7 (5-7) 6-3 7-6 (7-5) at London's O2 Arena against Djokovic and book a place in the semi-finals for the first time in his career.
Djokovic, who is bidding to equal Roger Federer's record of six titles and claim back top spot from Nadal, must now face the Swiss maestro in a straight shoot-out for the second semi-final spot on Thursday evening.
Djokovic came to London having won the Paris Masters for the fifth time and he started the tournament on fire, crushing Italian Matteo Berrettini for the loss of three games on Sunday. But the second round-robin clash with Thiem, a five-time winner on the Tour this year, proved to be a thriller.
He made an explosive start to lead 3-1, but Thiem, who has flourished this year since hiring coach Nicolas Massu after a slow start to the season, responded by playing perfect offensive tennis to break back without losing a single point.
The Austrian then saved three break points to hold for 3-3 in a top-quality duel full of thrills and spills, and it was fitting that the set was decided by a breaker.
The Wimbledon champion went on to inflict Thiem's eighth tie-break loss in succession despite some jaw-dropping winners from the 26-year-old.
Thiem, who took full advantage of an unusually sluggish Federer to make a flying start to the competition, responded in stunning fashion to break Djokovic at the start of the second set and he maintained his focus to keep the 16-time Grand Slam winner at bay to send a fabulous contest into a deciding set.
The momentum was well and truly with Thiem, who continued to bludgeon the ball en route to another early break followed by a mammoth hold. But his cavalier approach cost him in the sixth game as the 32-year-old hit back to restore parity.
With heart-in-the-mouth tennis being played at a pulsating rate, this classic contest looked to have been settled when a resilient Thiem broke to love for a 6-5 lead, but in a match of twists and turns, nerves got the better of him and a wayward game led to a third-set tie-break.
Djokovic surged into a 4-1 lead only to see Thiem recover and claim a monumental victory after two hours and 50 minutes, winning one of the best O2 matches in many a year.
Stats of the match
Djokovic has now lost more deciding sets (eight) than he's won (seven) in 2019.
Thiem: I was in the zone
Novak ready for Fed
What's next...
It's the big one on Thursday evening as Djokovic faces Roger Federer in a repeat of their epic Wimbledon final in which the Serb saved two championship points in the longest singles final at the All England Club to retain his title.
Federer has failed to reach the semi-finals only once in his previous 16 appearances, in 2008, but he has to beat Djokovic in his final group match to advance.
Thiem takes on Matteo Berrettini in the other match knowing he is already through to the semi-finals for the first time while the Italian debutant is out.
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