Cilic was going for back-to-back ATP World Tour titles before the 6-2, 6-2 loss to the fifth-seeded Nishikori. Cilic claimed his ninth career title two weeks ago in Zagreb, Croatia, and had won nine of 11 matches this year.
Nishikori's next opponent is Australia's Marinko Matosevic, who rallied for a 6-7 (6), 6-3, 6-4 victory over No. 7 seed Alexandr Dolgopolov. Feliciano Lopez of Spain beat American Jack Sock 6-4, 6-3 and next plays Denis Istomin of Uzbekistan, a 6-4, 7-6 (5) winner over American Michael Russell on Friday night.
Cilic's loss left the tournament without any of its top four seeds in the semifinals. No. 2 seed Milos Raonic, third-seeded John Isner and fourth-seeded Sam Querrey were eliminated by the second round.
Nishikori reached his second semifinal of the year. He reached the same round in Brisbane.
"It was a perfect match for me," Nishikori said. "Everything went well, my serves and my groundstrokes."
Nishikori played nearly flawless tennis after breaking Cilic to go up 3-2 in the first set. He broke Cilic twice in the second set to take a 4-1 lead and did not face a break point. Nishikori smashed a forehand winner on match point.
"I was struggling on my service games and not putting enough first serves on the court," Cilic said. "It's one of the crucial parts of my game and today it wasn't working."
Lopez, who advanced to the quarterfinals after No. 6 seed Tommy Haas withdrew because of illness, earned his 300th career win. He is now 300-293 all-time.
"It's a good achievement, and that means that I have been able to play for a long time, which is the only way to achieve 300 wins," Lopez said. "So I was really lucky to avoid a lot of injuries in my career, and this is basically the reason why I reached this number."
Lopez watched some YouTube videos of Sock to prepare for his first meeting with the 20-year-old.
Lopez served up seven aces and saved four of five break points against the American. Sock was up 2-0 and 40-0 in the second set before losing five straight points. Lopez broke Sock again to go up 4-3, held serve and then broke the American again to advance.
"It gave me a lot of confidence," Lopez said of breaking Sock in that third game.
Istomin improved to 3-1 against Russell in four career matches, but the 34-year-old American provided a stiff challenge in a match that took 1 hour, 37 minutes.
Istomin served up eight aces and broke Russell five times. Istomin grabbed a 4-1 lead in the tiebreaker before Russell closed to 6-5, only to hit a forehand long on the final point.
Erakovic is seeking her first career WTA title after reaching two finals, last year in Memphis and 2011 in Quebec.
Lisicki, the No. 3 seed, is playing in Memphis for the first time since 2009 when she reached the quarterfinals.
The Erakovic-Lisicki match will mark the final women's tournament held in Memphis. The event, which moved to Memphis in 2002, is moving to Rio de Janeiro in 2014.