Tennis: Thiem reaches ATP Finals semis as Djokovic and Federer face shootout
In the standout match so far at London's O2 Arena, the Austrian fifth seed took the fight to the world number two, recovering from losing the first set to triumph 6-7 (5/7), 6-3, 7-6 (7/5)
Earlier, Federer put himself back into the mix at the end-of-season event with a 7-6 (7/2), 6-3 win against Italian debutant Matteo Berrettini in their round-robin clash.
Forced to play on the limit, he hit 50 winners compared to Djokovic's tally of 27 but also racked up 44 unforced errors.
In a captivating first set lasting more than an hour, Djokovic, last year's beaten finalist, drew first blood, breaking in the fourth game, but Thiem responded immediately, breaking back with a rasping backhand down the line.
Thiem then broke in the first game of the deciding set and stayed strong to hold under severe pressure.
He then broke Djokovic to love to earn a chance to serve for the match but stumbled and the decider went to a tie-break.
"Novak was in great shape and I had to do something special," he added.
Third seed Federer had put himself under the cosh by losing his opening round-robin match to Thiem.
Federer, 38, will face Djokovic on Thursday in a repeat of July's marathon Wimbledon final, during which he squandered two championship points on his own serve.
"A lot of water has gone under the bridge since then.
On Monday, top seed Nadal lost his opener in Group Andre Agassi to defending champion Alexander Zverev while Stefanos Tsitsipas beat Daniil Medvedev.Read more: CNA
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