On Monday at the ATP World Tour Finals in London, the Japanese baseliner didn't let his lead slip. Indeed he thumped a listless Wawrinka 6-2 6-3 in 67 minutes to open play in the John McEnroe group.
"I felt very comfortable out there," Nishikori told reporters. "Started very good, from the first game. So I was very confident.
"I saw that he wasn't playing his best and he was missing. A lot of unforced errors. I saw a lot of opportunity today so I tried to be aggressive."
Later Monday in the same group, Andy Murray plays his first match as the official No. 1 when he faces Croatia's Marin Cilic. Murray, a Scot who resides near London, is sure to have the crowd behind him at the O2 Arena.
Nishikori is one of only two men to defeat the surging Murray since the start of September -- Juan Martin del Potro being the other. He is a flashy shot-maker who Roger Federer, for one, said he likes to watch.
But Nishikori didn't have to produce his best to see off Federer's fellow Swiss and good pal.
Wawrinka suffered from a sore knee recently and it likely played a part in his poor performance. He averaged nearly two unforced errors per game and his often breathtaking one-handed backhand severely misfired: He struck two backhand winners, making 15 unforced errors.
Yet the world No. 3 didn't apportion too much blame to the knee.
"I was feeling better on the court so I thought I could play at a better level today," Wawrinka told reporters. "I was expecting a good match. Didn't happen."
The fifth-seeded Nishikori broke in the fifth game of each set to take control. His break in the second set particularly stung Wawrinka, given the latter led 40-15.
It appeared Wawrinka might have awoken when he ripped a forehand winner down the line from outside the doubles alley in the eighth game of the second set but later in the game, he missed a second-serve return long and ballooned another second-serve return.
Wawrinka will take heart in knowing that he lost his first match last year, too, to Rafael Nadal at the World Tour Finals before making the semifinals. He has never failed to make the last four in his three previous appearances at the event.
But he can ill afford a repeat of Monday.
"I'm sure I still have something inside me to play some great tennis before the end of the year," said Wawrinka. "So I'm going to try everything for that in the next match. I'm going to do what I need to do tomorrow to get ready for trying to play better in two days."
Murray carries a 19-match winning streak into his clash with Cilic but lost to the 2014 US Open winner in Cincinnati in August.
The player he replaced atop the rankings, Novak Djokovic, began his tournament by beating Dominic Thiem on Sunday. Djokovic -- the four-time defending champion -- is assured of finishing the year as No. 1 if he wins one more round-robin match and then the title.