MEMPHIS, Tennessee (AP) — Kei Nishikori of Japan successfully defended his title at the U.S. National Indoor Tennis Championships, beating Ivo Karlovic of Croatia 6-4, 7-6 (0) on Sunday.

Nishikori won his fourth career title along with the $103,100 winner's check and 250 points on the ATP Tour by making this event at The Racquet Club his first successful title defense. He became the first back-to-back winner since Tommy Haas won in 2006 and 2007.

Karlovic came in having not been broken even once this week and looking for his sixth career title. The big-serving Karlovic and his 20 aces couldn't counter Nishikori's precision.

Nishikori saved both break points he faced while winning 86 percent on his first serve (38 of 44).

Karlovic had gone 42 games at The Racquet Club without being broken, and Nishikori broke him in the third game of the first set. Karlovic kept going to the net and 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 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.

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 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. 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.