Stefanos Tsitsipas beat six-time champion Roger Federer to reach the final of the ATP Finals in London.
The Greek, making his debut at the event aged just 21, won 6-3 6-4.
Federer, 38, was aiming for a record-extending seventh title but was undone by Tsitsipas, just as he was at January's Australian Open.
Defending champion Alexander Zverev and fifth seed Dominic Thiem meet later for the chance to play Tsitsipas in Sunday's final.
Federer was supreme in beating great rival Novak Djokovic to reach the semi-finals but was well below his best on this occasion and was left rueing a host of missed opportunities on crucial points.