Roger Federer produced his best performance yet at the ATP Finals to overcome Kevin Anderson and ensure his place in the semifinals in London.
The Swiss world No. 3, aiming for his 15th semifinal in 16 appearances at the season-ending event, was dominant on serve and showed moments of absolute class to win 6-4, 6-3 at the O2 Arena.
Despite the loss, Anderson qualified alongside Federer after Dominic Thiem defeated Kei Nishikori in the afternoon session.
Both players started in ominous form on serve but that soon changed in the opening set, as a series of breaks of serve brought the match to life.
Federer -- a six-time ATP Finals champion -- was first to strike against the head, racing to 0-40 on the Anderson serve at 3-3 before the South African gave the game away with a double fault.
It was Anderson's first dropped service game all week, but the world No. 6 responded instantly by staying alive in what looked a routine Federer service game to force advantage and get back on terms.
But the South African's serve then deserted him again and Federer broke to love with a classy backhand drop volley to make it eight points lost in a row for Anderson on serve.
In a dramatic first set there was still time for Federer to almost let Anderson back in, falling to 0-40 at 5-4, but the 20-time Grand Slam champion showed his mettle to save all three break points and close out the set, confirming his place in the semifinals.
That seemed to relax Federer, who produced some classic tennis in the second set.
His variation, including the use of drop shots to lure Anderson into the net before ripping winners past him time and again, exerted pressure which Anderson failed to cope with.
At 3-3 Federer made the breakthrough, profiting from two errors on the Anderson backhand to move closer to victory, and he closed out the match with another break of the Anderson serve to finish top of the Lleyton Hewitt group and make a semifinal encounter with Novak Djokovic -- who had already qualified -- less likely.
"The first match was tough against Kei [Nishikori]," said Federer on court after the match.
"I never got going. With my back against the wall maybe it is easier for me to play. I fought hard, spoke a lot with my team to come up with a good game plan against Dominic [Thiem] and Kevin also today.
"I am happy it all worked out in the end. I played some very good tennis today. Kevin has had a very good year. So have the other two guys so it has been a fun group. I am thrilled."
Earlier Thiem beat Nishikori 6-1, 6-4 to record his first win at the 2018 ATP Finals.
The Austrian world No. 8, already beaten by Anderson and Federer in his first two round-robin matches, broke twice in the first set and took advantage of Nishikori's 42 errors to keep his faint hopes of qualification alive.
But both Thiem and Nishikori, who had promised so much by beating Federer in the opening match, are now out as the Swiss former champion and Anderson, the first South African to make the semifinals, advance.