Kei Nishikori piled the pressure on Andy Murray by beating stand-in David Ferrer at the ATP World Tour Finals in London.
Nishikori's 4-6 6-4 6-1 victory means Murray must defeat Roger Federer in straight sets later on today to pip the Japanese to a semi-final spot.
Ferrer came in as a late replacement for Milos Raonic, who pulled out with a thigh injury, and put in a highly creditable performance despite knowing before the start he had no chance of going through.
The set he won secured Federer's place in the last four, although there was still plenty to play for the world number two with the Group B winner likely to avoid defending champion Novak Djokovic until the final.
Raonic, who failed to win a set on his debut at the tournament, made his announcement less than two hours before the match and revealed he suffered the injury during Tuesday's defeat to Murray.
"I have a slight tear in my quad and a large area of swelling," he said.
"It would be unfair to fans if I was to go out and play a mediocre match. If I was to step out on court I could lose up to six to eight weeks. That was a significant factor in my decision."
Ferrer only missed out on a place in the top eight at the very last minute after Raonic defeated Federer at the Paris Masters and he lost to Nishikori.
The Spaniard, one of the game's great street-fighters, rarely needs any extra motivation but that might have provided it, along with a significant amount of prize money and 200 ranking points.
Nishikori, who had his own injury worries after receiving treatment to his right wrist against Federer, had won five of their previous eight meetings, including all three this season, but every one had been extremely close.
It quickly became clear this would be the most competitive match of the week so far, with the pair trading fizzing groundstrokes.
Having prepared to face a big server, this was a completely different scenario for Nishikori, but he began well and was in control of most of the rallies.
A forehand winner earned him the first break of serve in the seventh game but Ferrer hit straight back and a double fault and smash into the net from Nishikori handed the opening set to the alternate.
But Nishikori put the disappointment behind him quickly by breaking in the opening game of the second set and an improved serving display from the fourth seed helped him keep Ferrer at arm's length.
The 24-year-old's groundstrokes were more accurate than his opponent's and carried more bite, and he was well on his way when he opened up a 4-0 lead in the decider.
Ferrer threatened a comeback but could not take any of four break points for 4-2 and Nishikori moved away again to win the first three-set match of the tournament.
