Kei Nishikori retains US National Indoor Championships

Kei Nishikori has retained his title at the US National Indoor Championships.

The Japanese number one beat Croatia's Ivo Karlovic 6-4 7-6 in the final on Sunday.

Top seed Nishikori claimed the opening set courtesy of a solitary break, as Karlovic lost a service game for the first time in this year's tournament.

There were no breaks of serve in the second set, which eventually went the way of Nishikori when he won the tie-break 7-0.