The 18-year-old Nishikori, 131st in the world rankings following his victory over James Blake on Sunday in Delray Beach, Fla., will face top-seeded Andy Roddick, a 6-3, 6-7 (3), 7-6 (5) winner over Chris Guccione.
Nishikori, the first Japanese player to win an ATP Tour title since Shuzo Matsuoka in Seoul in 1992, was looking forward to a match against Roddick who was in the stands.
"I think I'm ready," Nishikori said. "I'll be facing another top 10 player. His serve is a big thing. He'll be the highest ranked player I've ever played."
Midway through the first set, Nishikori saw Roddick watching.
"When I was down 4-3, I saw him in the stands," Nishikori said. "I got really nervous. He was sitting next to my coach. I wanted to play Roddick. That's why I won."
"I saw him play a little bit last week," Roddick said of Nishikori. "I'd be lying if I said I knew a lot about him. I saw him play tonight. He looks like he's full of confidence and he has a lot of momentum going for him.
"I've begun asking around about him. I hope to talk to James [Blake] tomorrow about their match together last Sunday," Roddick said.
After winning the first set handily, Roddick struggled through the next two sets. For every point Roddick won on his serve, Guccione matched him.
Guccione turned Roddick's game of aces against him with a 20-9 edge in the match.
"I thought he passed really well tonight," Guccione said. "I thought the match was a good sign for me. It was against a top 10 player and it showed that I'm getting closer to them."
"I thought it came down to a couple points tonight," Roddick said. "I'm glad I had something to show for it at the end of the match."
Guccione won the second set when he ran together seven straight points in the 6-6 tiebreaker.
"I played a sloppy double fault," Roddick said. "And, then, he put together a little bit of a run of points on me. It was also the kind of match that you can play horribly and win and you can play well and lose."
Roddick has won twice at the SAP Open and owns a 21-4 record in San Jose.
In other afternoon matches, fourth-seeded Radek Stepanek of the Czech Republic edged Bobby Reynolds 7-5, 7-6 (5); Robby Ginepri upset eighth-seeded Kristof Vliegen of Belgium 6-2, 6-3; and Guillermo Garcia-Lopez of Spain beat Benjamin Becker of Germany 3-6, 7-6 (3), 6-4.