|2019 Nitto ATP Finals|
|Venue: O2 Arena, London Dates: 10-17 November|
|Coverage: Watch live coverage of one match per day on BBC TV, BBC iPlayer and online; Listen on BBC Radio 5 Live Sports Extra; Live text on selected matches on the BBC Sport website and app. Click here for Live Guide.|
Stefanos Tsitsipas will play Dominic Thiem for the title at the ATP Finals on Sunday after beating six-time champion Roger Federer in London.
The Greek, making his debut at the event aged just 21, won 6-3 6-4.
Swiss Federer, 38, was aiming for a record-extending seventh title but was undone by the nerveless Tsitsipas, just as he was at January's Australian Open.
Later, Austrian fifth seed Thiem saw off defending champion Alexander Zverev 7-5 6-3 at the 02 Arena.
Thiem, 26, and Tsitsipas will be competing in their first final at the season-ending tournament and aiming for the biggest title of their careers.
Asked what fans could expect from the final, Thiem replied: "A one-handed back-handed final, I guess! We are both offensive players, he is very attractive to watch, I love to watch him and I am looking forward to playing him again."
Tsitsipas is the youngest finalist since Juan Martin del Potro in 2009, while it will be just the third final since 2002 not to include Federer, Novak Djokovic or Rafael Nadal.
Federer was supreme in beating great rival Djokovic to reach the semi-finals on Thursday but was well below his best on this occasion and was left rueing a host of missed opportunities on crucial points.
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Tsitsipas wins battle of the generations
The 17-year age gap between the two players is the biggest in the history of the season-ending championships.
With the London crowd heavily in favour of Federer, Tsitsipas played the better tennis, particularly on the big points.
The Greek saved break point in the very first game before clinching Federer's opening service game minutes later with a forehand winner.
In total, Federer had six break points in the opening set but failed to take any as Tsitsipas held firm and the 20-time Grand Slam champion gifted points with errors.
Serving for the set at 5-3, Tsitsipas fought off two of those break points but saw six set points of his own come and go in a marathon game before clinching the opener on his seventh.
Federer's errors gave his opponent an further early break in the second set, only for the Swiss to finally convert a break point a game later at the 10th attempt.
But, after the crowd roared in hope of a comeback, Tsitsipas crunched another forehand winner to seal Federer's service game.
Typically, Federer had two more break points when Tsitsipas served for the match but the Greek closed out the win with some big deliveries.
Tsitsipas was aged just five when Federer won his first Wimbledon title in 2003 and said it was a "dream come true" to beat him in London.
"I grew up watching Roger here at the ATP Finals and Wimbledon and other finals," he said.
"I wished one day I could face him and today I'm here living the dream.
"I remember myself being one of the kids here watching the event and I could never picture myself here. But it can happen."
He was already the youngest player to have recorded wins against Federer, Djokovic and Nadal but this victory strengthens further his standing as one of the best young players in the game.
In his breakthrough year the world number six reached the Australian Open semi-finals and won two other titles.
Federer pays for his errors
For Federer the tournament ends in disappointment after an underwhelming performance, where he failed to convert 11 of his 12 break points.
He lost his opening match of the round-robin stage against Thiem but after beating Matteo Berrettini and then outplaying Djokovic he was tipped against to win his first title at the season-ending championships since 2011.
Against Djokovic he hit just six unforced errors in the entire match. Against Tsitsipas he made 26.
His usually impeccable forehand let him down consistently, with 17 of those errors coming off that wing, while he also missed a series of second-serve returns.
At deuce in what proved to be the final game of the match, a wayward forehand and the shake of the head that followed summed up his performance.
"No doubt I had my chances," Federer said.
"The spells where things were not working well, they were pretty bad.
"At this level, you just can't have it happen, so that was pretty disappointing today."
The Swiss' season is now over. He will turn 39 next year but is expected to return again, looking to add to his 103 career titles - and close on Jimmy Connors' all-time record of 109 - and his haul of 20 Grand Slams.
Thiem puts friendship aside to reach final
Thiem did not hit the heights of his impressive round-robin wins over Federer and Djokovic but his victory means the two most impressive players across the week have reached the final.
The Austrian was on the back foot for much of the first set against Zverev, struggling to make an impact on his opponent's service games with the German's first-serve percentage at 84%.
But Zverev faltered at the crucial moment, double-faulting on set point to give Thiem the lead and flinging his racquet across the court in frustration.
Thiem capitalised on more mistakes from the defending champion in the second set with Zverev hitting another double fault and missing two smashes in a game that proved to be the only other break of the match.
"It is unreal to me and to beat the defending champion, a very good friend and unbelievable player - it is an unbelievable achievement. I'm very, very happy," Thiem said.
The final comes at the end of a fine year for Thiem, who has won five titles - the joint most on tour alongside Djokovic.
He has been often talked about as a future clay-court Grand Slam champion, and reached a second successive final at the French Open this year, but this year has progressed to become a threat on other surfaces.
This final comes after he won on the hard courts of Indian Wells in March and this win also means he will overtake Russia's Daniil Medvedev as the world number four next week.