Six-time champion Roger Federer beat Kevin Anderson 6-4 6-3 to secure his place in the semi-finals of the ATP Finals for the 15th time.
Federer's victory means he tops his group to boost his chances of avoiding world number one Novak Djokovic in the last four in London.
The Swiss, 37, looked out of sorts when he lost his first match on Sunday but played much better against Anderson.
South African Anderson had already won two matches to reach the semi-finals.
Top seed Djokovic is already through to the semi-finals and can clinch top spot in his group by beating Marin Cilic on Friday.