Defending champion Kei Nishikori of Japan advanced to the quarter-finals of the ATP’s US Indoor Championships on Wednesday by defeating Germany’s Benjamin Becker 6-4 6-4.

Nishikori, the top seed for the first time in an ATP event, led 4-2 in the second set but was broken in the eighth game by Becker to level the set.

But the German double-faulted away a break in the ninth game and Nishikori held for the victory after 84 minutes.

The 17th-ranked Japanese star denied Becker on 5-of-8 break points while winning the same amount of break chances from the German.

Nishikori, who like all top-four seeds enjoyed a first-round bye, also connected on 65 per cent of his first serves to 48 per cent for Becker.

Nishikori, who scored three points to lead Japan over Canada in a Davis Cup tie earlier this month, reached a last-eight match-up on Friday against Russian Alex Bogomolov, who defeated American Ryan Harrison 6-3 2-6 6-2 in another second-round match on Wednesday.

Also reaching the quarter-finals were Americans Jack Sock, who bounced Czech Jiri Vesely 6-2 6-1 and Michael Russell, who ousted countryman Tim Smyczek 6-2 7-6 (7-5).

Russell will next face the winner of a Thursday match-up between Australian third seed Lleyton Hewitt and Cypriot wildcard Marcos Baghdatis.

Either Spanish second seed Feliciano Lopez or Croatia’s Ivo Karlovic will face Sock in the quarter-finals.

US qualifier Denis Kudla beat compatriot Donald Young 6-1 6-1 to book a second-round match on Thursday against Taiwan fourth seed Lu Yen-Hsun.