Roger Federer plays a backhand in his singles match against Matteo Berrettini at the ATP World Tour Finals on Tuesday at The O2 Arena in London. Photo: AFP

 
Novak Djokovic suffered defeat in a thriller against an inspired Dominic Thiem at the ATP ­Finals on Tuesday and now faces a showdown with great rival Roger Federer to decide who will reach the last four at the season-ender.

Swiss Federer shrugged of defeat by Austria's Thiem in his opening group match to comfortably beat Italian Matteo Berrettini 7-6 (7/2), 6-3 in the day's opening singles.

But the real drama at The O2 Arena came later when Thiem beat Djokovic 6-7 (5/7), 6-3, 7-6 (7/5) in the standout match of the tournament so far.

World No.5 Thiem blazed 50 winners in a sensational display of aggressive hitting to claim his second group win and seal his first semifinal in his fourth appearance.

Whatever his result against the already-eliminated Berrettini the Austrian will progress, but only victory will do for Djokovic and Federer as they clash for the 49th time and first since Djokovic won an epic Wimbledon final in July.

While Federer is trying to avoid only his second group stage exit in 17 appearances at the year-ending showpiece, the stakes will be even higher for Djokovic.

Defeat would not only halt his bid to equal Federer's six titles but would also mean he cannot end the year ranked No.1. Djokovic must reach at least the final to overhaul Rafael Nadal and equal Pete Sampras' feat of finishing a year ranked No.1 on six occasions.

He will have to recover quickly after being stretched to breaking point by Thiem in a match lasting nearly three hours.

"We go head to head with Roger. Winner goes to semis. Loser doesn't qualify. As simple as that," Djokovic, who led 4-1 in the final-set tiebreaker against Thiem, told reporters.

The 26-year-old Thiem is the first Austrian to reach the semifinals of the tournament.

Federer was pushed hard by Berrettini in the first set as the Italian powerhouse showed the kind of form that has propelled him into the world's top 10.

But once Federer changed gear to claim 10 of 11 points to breeze through the first set tiebreak and break to love at the start of the second his 58th victory at the tournament was never in doubt. The 38-year-old's thoughts then turned to Djokovic.

"We'll find out, but I think it's all flushed away from my side," the 20-time Grand Slam winner said when asked if the memory of his Wimbledon loss to Djokovic was still bitter.

"I think we both can take away some confidence from the match. Him obviously a lot. Me maybe a tad bit less.

"But at the end of the day, I didn't hope I was never going to play him again. Actually, it's good for me to play him again, and maybe that all helps to get a chance to get him back or whatever it is. Personally, excited to play Novak on Thursday."