Roger Federer knocked Japan's Kei Nishikori out of the ATP Tour Finals and guaranteed top spot in his group with a 7-5, 4-6, 6-4 victory on Thursday.
Federer had already clinched a record 13th Tour Finals semifinal place after ending Novak Djokovic's three-month, 23-match winning run on Tuesday.
But the 34-year-old still needed to beat Nishikori to be certain of advancing as the group winner and he finished the job in two hours and 10 minutes of gruelling action to secure his third successive victory at the prestigious season-ending event at London's O2 Arena.
Federer, who has won the Tour Finals a record six times, will face either Andy Murray or Stanislas Wawrinka in the last four on Saturday.
Murray and Wawrinka clash on Friday to determine the second qualifier from the other group, which has already been won by Rafael Nadal.
On Wednesday, French Open champion Wawrinka defeated David Ferrer 7-5, 6-2 to pack him off from the tournament. Ferrer was 4-1 up in the first set and had set point when serving at 5-3 but double-faulted. Wawrinka took advantage, broke and never looked back.
The Swiss broke again to go 6-5 up after another Ferrer double fault, then won the next game to love to take the set. Wawrinka then motored through a second set in which he hit 19 winners to two from Ferrer.
Nishikori's last meeting with Federer had also been at the Tour Finals 12 months ago and ended in a heavy defeat for the Japanese star. This time the 25-year-old put up much more of a fight, but once again he was denied a third career win over the player he grew up idolising.
Nishikori has struggled with a host of injuries during a disappointing 2015 and, still troubled by fitness issues this week, he could have allowed his season to fizzle out when a wayward backhand gifted Federer a break in the fourth game of the first set. But Nishikori is too spirited a competitor for that and he broke back in the next game.
Nishikori broke again in the seventh game, converting with a ferocious crosscourt winner that left his opponent rooted to the spot. Yet as suddenly as Nishikori had seized the advantage, he found it vanished just as quickly as Federer took advantage of a sloppy service game to break again.
Unable to find a solution to Federer's pressure, Nishikori finally wilted in the 12th game with a pair of unforced errors surrendering the first set.
It looked all over for Nishikori when he trailed 4-1 in the second set, but to his credit he came back fighting and started to go for winners with almost every shot.
His cause was aided by a drop in Federer's intensity and the former US Open finalist made him pay when he broke back in the seventh game.
Now Nishikori was the aggressor and he broke again for a 5-4 lead before closing out a set that looked beyond him moments earlier. Federer regained the initiative when he broke for a 3-1 lead in the final set, yet the indefatigable Nishikori clawed his way out of trouble with a break in the seventh game.
However, Federer wouldn't be denied and he unloaded a booming smash to finally see off Nishikori on his first match point.
Bopanna-Mergea lose
Already through to semifinals, Rohan Bopanna and Florin Mergea lost to Italian pair of Fabio Fognini and Simone Bolleli in their last group match of the doubles event 4-6, 6-1, 5-10 on Thursday.
Bopanna and Mergea topped the group despite the loss.
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