Karlovic kept going to the net with Nishikori popped a backhand past the Croat into the corner for the advantage after pushing the game to deuce. Then Nishikori broke him with a backhand passing shot past Karlovic for a 2-1 lead.
Nishikori played very efficiently, not losing a point off his serve until the eighth game of the first set when up 30-0.
The top seed fought off two break points before serving up an ace for the advantage, and Karlovic hit a return long as Nishikori held serve for a 5-3 lead. Karlovic didn't make it easy as Nishikori finally converted on his fourth set point to take the first set in 40 minutes on a Karlovic backhand into the net.
In the second set, Karlovic found himself again having to hold his serve. He faced five break points and double-faulted twice before finally taking advantage of a Nishikori return that went wide before Karlovic won the game with an overhead shot.
Nishikori dominated the tiebreaker from the start, hitting a passing shot past Karlovic for the mini-break. Karlovic hit forehands long on the next two points.
"He was really fast, and he hit two like mishit and it was in," Karlovic said. "And I just didn't have my first serve in. That's where he really took the opportunity."
Nishikori beat Karlovic with a passing shot off his forehand putting up him 5-0 that he celebrated with a fist pump. Then Nishikori finished off the match with a forehand into the corner before Karlovic hit a return long.
Follow Teresa M. Walker at www.twitter.com/teresamwalker