Nishikori played very efficiently, not losing a point off his serve until the eighth game of the first set when up 30-0.
He 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 did not 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. Nishikori beat Karlovic with a passing shot off his forehand putting up him 5-0 that he celebrated with a fist pump.
Nishikori finished off the match with a forehand into the corner before Karlovic hit a return long.
Reuters, BUENOS AIRES
David Ferrer won his third consecutive Buenos Aires ATP Tour clay-court title and the sixth in a row by a Spaniard, when he beat Italian Fabio Fognini 6-4, 6-3 in the final on Sunday.
Ferrer had not planned to return this year, but stood in at the last minute following world No. 1 Rafael Nadal’s withdrawal through injury.
The 31-year-old was to climb to fourth in the world rankings yesterday.
“This title was very important to me after seven finals without a victory,” Ferrer said after claiming his 21st career win.
Fognini had won 10 matches in a row, including helping Italy beat Argentina in the Davis Cup and victory in the Chilean ATP tournament at Vina del Mar the previous weekend.