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.

American Eric Butorac and Raven Klaasen of South Africa beat Bob and Mike Bryan 6-4, 6-4 for the doubles title. It's the second time this year Butorac and Klaasen have beaten the Bryans after knocking them out of the Australian Open in January.