Dominic Thiem of Austria, right, is congratulated by Stefanos Tsitsipas of Greece after winning the China Open tennis tournament last month. Photo: Mark Schiefelbein/AP

LONDON - Greece's Stefanos Tsitsipas will face Austrian Dominic Thiem for the season-ending ATP Finals title after battling past Roger Federer 6-3, 6-4 in Saturday's semi-finals. 

Meanwhile, Thiem ended the run of Germany's defending champion Alexander Zverev with a 7-5, 6-3 victory in the other semi-final.

"It is unreal to me and to beat the defending champion, a very good friend and unbelievable player - it is an unbelievable achievement," said Thiem. 

"This is a big, big dream coming true for me." 

Thiem now looks forward to playing one of the most exciting talents on the tour as they will both play in their first ATP Finals showpiece.

"We are both offensive players," said Thiem.

"He is very attractive to watch, I love to watch him and I am looking forward to playing him again."

Earlier, the 21-year-old Finals debutant Tsitsipas saved 11 of 12 break points, which Federer missed in an error-strewn performance. 

Serving for the first set advantage at 5-3, Tsitsipas struggled to close it out but on his seventh set point, he moved ahead after a powerless return from Federer dropped into the net. 

Tsitsipas broke twice in the second set, despite Federer finally converting a break point at the 10th time of asking for 2-2, only to fall 4-2 behind after Tsitsipas rallied into a commanding position against his tennis idol.

"I'm so proud of myself today," said Tsitsipas. 

"It is a mental struggle and I'm proud how many I saved today, I was trying not to give an easy time to Roger. 

"I grew up watching Roger here at the ATP Finals and Wimbledon and other finals. I wished one day I could face him and today I'm here living the dream."

The world number six, who had beaten Federer at the Australian Open in January, saved two break points serving for the match at 5-4 before completing a career highlight victory with an ace in 1 hour 36 minutes. 

In the evening session, Zverev missed two break points at 1-1 and 2-2 in the first set before handing the lead to Thiem with a nervous double-fault in the 12th game.

Thiem, who qualified for the season-ending event for a fourth straight year, rallied to break at 4-2 in the second set before completing victory with a triumphant forehand after 1 hour 34 minutes. 

Federer and Tsitsipas created the biggest age gap of 17 years in an ATP Finals contest, and after a momentous win over Novak Djokovic Thursday, Federer threw away many chances. 

The six-time Finals champion saw his first break point in the opening game sail away with a long return and that would be the start in a plethora of 26 unforced errors. 

"No doubt I had my chances. Yeah, don't know exactly why it went the way it did," said Federer. "I had some good spells, but the spells where things were not working well, they were pretty bad." 

An undeterred Tsitsipas lashed back at Federer for his slow start and fired down a forehand to break and then extend the gap to three games without reply by hitting his first ace.

The ninth game of the first set could've gone to either player as they were locked in a deuce battle where Federer missed another break point pair.  

Tsitsipas had a torrid time completing the frame as he saw six set points fly by but on the seventh, Federer wilted. 

The 20-time major winner Federer, who was aiming for a record-extending 11th season-ending final, was broken again in the second set to trail 2-1. 

Federer earned a break to bring the scores level, but only to fall behind again as Tsitsipas collected a third break with a crushing forehand winner to edge 3-2 in front. 

"I'm frustrated I couldn't play better," Federer said.

"When I did and fought my way back, I threw it away again. He (Tsitsipas) did come up with the goods when he had to, and he was better than me today."

Federer shook his head after missing two break points at 5-4 before sending a long backhand to open up match point for Tsitsipas.

After his sixth and final ace skipped past the former world number one, Tsitsipas dropped his racket in disbelief, realising he had conquered his hero twice at the start and end of a career-defining year. 

DPA