Roger Federer will take on Novak Djokovic for the title after beating fellow Swiss Stan Wawrinka at the ATP World Tour Finals.
The six-time champion saw off his compatriot 7-5 6-3 at London's O2 Arena to reach the final for the ninth time.
Djokovic earlier beat Rafael Nadal 6-3 6-3 and remains on track for a fourth straight season-ending title.
Federer and Djokovic were due to play in last year's final but the Swiss pulled out with a back injury.
Djokovic dismantles rejuvenated Nadal
The world number one was at his clinical best to draw level at 23-23 in career meetings with Nadal and claim his eighth win in their last nine matches.