A loss, too, would have diminished Novak Djokovic's chances of reaching the last four -- and the world No. 1 has been Federer's major road block this year, even if the Swiss toppled the Serb Tuesday to end his hefty 23-match winning streak.
But Federer is too much of a pro and respects the game too much to take his foot off the accelerator, especially when a large crowd is rooting for him, as was the case at London's O2 Arena.
History tells us that Federer doesn't stumble in situations where he had advanced or come close to it with a match to spare, since the 34-year-old had finished the round-robin stage of the World Tour Finals with a perfect 3-0 record on eight previous occasions.
The record six-time champion made it nine by defeating, and eliminating, Nishikori 7-5 4-6 6-4 in a rousing affair -- despite what had to have been one of the worst misses of his career.
"It's just another match, a really important one for me, to move on with a good feeling," Federer told reporters. "There's points on the line and prestige, wanting to beat Kei, winning at the O2.
"I want to do well here."
Federer awaits the winner of Andy Murray's clash Friday with Federer's buddy, Stan Wawrinka, in the semifinals.
Either way it figures to be an enticing contest. The lone time Federer failed to reach the semifinals in 14 attempts at this event, Murray was the man who knocked him out in Shanghai in 2008 in a bruising match.
And last year in London, Federer edged Wawrinka, his Davis Cup teammate, in a thriller that came with added spice after Federer's wife, Mirka, taunted the Swiss No. 2 late in the third set.
Federer withdrew from the final against Djokovic with a back injury but said his body is fine heading into this weekend.
"I'm off before the semis, which is huge," said the 17-time grand slam champion. "Now you're looking at potentially a maximum of two more matches and the season's over.
"It's easy for the mind now."
Djokovic simply needed to win a set later Thursday against Tomas Berdych to progress and set up an encounter with a rejuvenated Rafael Nadal, Federer's first serious rival. He did that and more, ousting the Czech 6-3 7-5 to bounce back from the loss to Federer.
Federer perennially dazzles with his varied, attacking game but Nishikori is no slouch when it comes to entertaining fans, either.
He lacks Federer's all-round approach but crunches flat balls hugging the baseline, not shy to change directions and hit down the lines.
Federer won the point, arguably, of the match late in the first set, after he had led by a break and then trailed by one. Federer lured the 2014 U.S. Open finalist to the net with a drop shot, which the speedy Japanese chased down. After Nishikori was forced to hit two volleys, Federer stroked a backhand winner down the line, prompting a thumbs-up from his foe.
It allowed Federer to get to deuce with Nishikori serving at 5-6, and two subsequent unforced errors gave the former a one-set advantage.
Cruising at 4-1 in the second, Federer inexplicably swooned, though, dropping 18 of the last 22 points in the set to prompt a third.
It was during the sequence that Federer miscued badly on a shot at the net. Forced to race to the net off a fortunate, dipping return at 4-4, 30-all, he swatted his cross-court forehand put away well outside the tram lines. No one quite believed what they saw.
The turning point of the third, perhaps, came in the third game, when Federer dug out of a 0-40 hole to hold for 2-1.
Nishikori stroked a half-slice, half-drive backhand winner down the line on the run in another moment that had the crowd gasping in appreciation -- but he was broken three points later to trail 3-1.
Federer found himself up 4-1 in a set once again but was pegged back by the resilient Nishikori, who won 70% of points returning second serves.
Back on serve at 3-4, the eighth game was highlighted by a brilliant Nishikori half-volley winner -- the contact point coming with both racket and ball behind him.
Last year's semifinalist ultimately imploded, hitting a fatal double fault on game point at 4-5, with Federer closing things out helped by a rasping forehand down the line.
"In the end it was a really close game," said Nishikori (1-2), the youngest player in the field at 25. "Just a couple of points that I didn't play quite well enough. It's a shame to lose like this."
Federer, the oldest participant, went 6-for-6 on break points. Perfect, as he was in the group stage.
Djokovic (2-1) wasn't in top form but the world No. 1 and three-time defending champion didn't need to be against Berdych (0-3), who slumped to 2-21 against the 28-year-old.
Djokovic's mild dip in form and Nadal's recent renaissance suggest their 46th tussle could be closer than their three head-to-heads this year, all won by Djokovic in straight sets.
"We've played so many times on different surfaces," Djokovic told the crowd after beating Berdych. "He's been playing some very good tennis the first couple matches here. Hopefully I'll be able to perform at my best, because that's what's needed to win against him."