Novak Djokovic completed a clean sweep of wins in the ATP Finals group stages when he dispatched Marin Cilic in straight sets on Friday night.
The match was already a dead rubber but Djokovic warmed up for the semi-finals with a 7-6 (7) 6-2 victory.
World number one Djokovic will face South African debutant Kevin Anderson, the man he beat in this year's Wimbledon final, in the other semi-final.
"Obviously I knew before the match that I had already qualified, and I knew who I'm playing, so it wasn't easy to approach this match with 100 per cent willingness to win it," said Djokovic.
"But this is a unique format and both of us wanted to win from the first point. It was a close first set which probably decided the match.
"(Anderson) is playing some great tennis, it's definitely his best season and he's continued doing that throughout this tournament.
"He's playing well, serving well, very aggressive, so I know what to expect."
