Roger Federer's enduring brilliance and his sheer hatred of losing allowed him to withstand a stunning Kei Nishikori fightback and remain undefeated at the ATP World Tour Finals on Thursday.
The 34-year-old Swiss maestro was on auto-pilot when he led 4-1 in the second set, having snatched an absorbing opener, but needed every shot in his formidable armoury to eventually claw out a 7-5, 4-6, 6-4 victory.
Japan's Nishikori had begun to look the more likely winner after reeling off five consecutive games to take the six-times former champion into a deciding set, but his stirring efforts were in vain as Federer squeezed home.
Federer wins Stan Smith Group with a 100 percent record -- the ninth time in 14 consecutive appearances at the elite event he has won all three of his round-robin matches.
World number eight Nishikori bowed out having only beaten Tomas Berdych, while favourite and defending champion Novak Djokovic will take the runners-up spot if he beats Berdych later at the O2 Arena, or avoids a straight sets loss.
"It was extremely difficult. It was a good match, with more breaks than we are used to. Kei is a very good returner, especially on the second serve. It was a very physical match, but I enjoyed it," Federer said on court.
"Kei was staying aggressive and going for his shots and that makes it hard. I had to hope that he would slip."
Federer will not know his semi-final opponent until Ilie Nastase Group is concluded on Friday with the match between Britain's Andy Murray and compatriot Stanislas Wawrinka.
Rafael Nadal has already won the other group.
Seventeen-times grand slam champion Federer had already sealed a semi-final spot before walking on court on Thursday, but there was never a chance of him letting up.
He jumped into a 3-1 lead before 26-year-old Nishikori began striking the ball with real venom, breaking twice in a row.
Federer squared the set at 4-4 with some rock-solid baseline play wearing down Nishikori.
Nishikori had a point to set up a tiebreak at 5-6 but after a superb cat-and-mouse rally, Federer threaded a backhand down the line. Federer pounced on set point, forcing an error.
Federer was in full flow in the second set, leading 4-1, but 2014 U.S. Open runner-up Nishikori refused to slink away quietly and took the next five games with some delightful shot-making.
His crucial break at 4-4 came courtesy of an ugly forehand swipe from Federer who hit the ball several metres out with the court at his mercy.
Federer looked tired and tetchy early in the third, but crucially hung in from 0-40 down to hold serve, before breaking to move 4-1 ahead.
Again Nishikori stormed back to 4-4, but this time Federer steadied down, found four first serves, then broke for the match with a winning smash.
Bryans edge cliffhanger to stay alive
Top seeds and defending champions Bob and Mike Bryan saved five match points to beat Jamie Murray and John Peers 6-7(5), 7-6(5), 16-14 in a cliffhanger and reach the semi-finals at the ATP World Tour Finals on Thursday.
The American twins looked out of it when they trailed 9-5 in the deciding championship tiebreak but used all their craft, and some luck, to claw back the deficit and qualify at the expense of the Anglo/Australian duo.
It was their 35th victory in the year-end championship -- taking them past the record they had shared with Daniel Nestor.
A nerve-jangling scrap ended when the Bryans converted their fifth match point -- but the match will haunt Peers who blazed a forehand long with the court gaping at 9-8.
The 37-year-old Bryans, who have won a record 109 titles together, have now reached at least the semi-finals of the season-ender for the 10th time, winning four titles.
They go through as runners-up in their group and will find out their semi-final opponents on Friday.
