Raonic struggled in his first two round-robin matches at the season-ending event, bowing in straight sets to Roger Federer on Sunday and Andy Murray on Tuesday.
The Canadian's powerful serve has been rather muted at the O2 Arena, with noticeable drops in both speed and first-serve percentage. Raonic was looking for solutions in Wednesday's practice session with coaches Ivan Ljubicic and Riccardo Piatti.
"Whatever the reason is, I have 24 hours to solve it," Raonic said. "I'm going to need to (fix it) if I want to have any hope. There's no way around that. It's something I've got to do."
Raonic entered the eight-man tournament brimming with confidence. He knocked off Federer for the first time at the recent Paris Masters before falling to Novak Djokovic in the final.
The 23-year-old Canadian admits he has been too passive on the court and needs to get back to dictating the play.
"I've got to be more aggressive," he said. "I have to go for more rather than just play relatively down the middle. I'm not stretching my opponents most of the time. I've got a terrible minus record (of) unforced errors to winners ratio at this moment. That's not how I'm going to give myself a possibility to win."
Raonic, from Thornhill, Ont., and Nishikori are both making their first appearances at the event. Nishikori, who's enjoying a breakout season, was the runner-up at the U.S. Open in September.
"I'm going to have to serve well, as always, that's key for me," Raonic said. "But I'm just going to have to be aggressive. I'm going to have to make Kei play on my terms. It's something I haven't been able to do so far these last two matches."
Raonic said he's using the same approach despite the tournament's unique format.
"You go out there and you treat every match like it's a single elimination tournament, there's no other way to do it," he said. "If you go out there hoping that you can lose a match and maybe still get through, things can only go poorly from there on. Obviously I think if I can put myself in this situation again, I hope I can do better.
"I need to go back to basics, focus on rhythm, focus on the balance of my body, the toss, make sure the basics are in place, just go through repetition doing the right thing. It's just as simple as that, but it takes time to get it right."
Nishikori enters with a 4-1 career record against Raonic. The Canadian's lone victory came at Wimbledon last summer.
Toronto's Daniel Nestor and Serbian partner Nenad Zimonjic also had the day off Wednesday. They opened with two losses and are longshots to qualify for the doubles semifinals.