The Swiss tennis star Roger Federer has advanced to the last four of the ATP Finals for a record-extending 15th time with a 6-4 6-3 victory over Kevin Anderson.
Six-time champion Federer needed only a set to advance and produced his best performance at the season-ending tournament to end the round-robin stages with a 2-1 win-loss record.
Dominic Thiem's 6-1 6-4 victory over Kei Nishikori earlier had left Federer with a modest task and ensured that Anderson became the first South African to reach the semi-finals.
Federer looked as if he was about to give up a break advantage for the second time in as many service games when he dropped to 0-40 attempting to close out the opening set at 5-4, but the 20-time grand slam champion found his first serve when it mattered most to hold.
Anderson hadn't faced a break point in winning his first two matches.
Federer, though, broke him twice more in the second set to claim top spot in the group and avoid a potential semi-final meeting with top-ranked Novak Djokovic.
Thiem claimed his first win of the week as he dominated throughout with Nishikori guilty of 41 unforced errors.
After describing his error-strewn loss to Federer as "unacceptable"', Thiem produced a performance much more in keeping with a player who reached the French Open final this year and who has now won 54 matches during the season.
The other group will be decided overnight, with Novak Djokovic already through.