Memphis - Defending champion Kei Nishikori of Japan advanced to the quarter-finals of the $647 000 ATP US Indoor Championships on Wednesday by defeating Germany's Benjamin Becker 6-4, 6-4.
Nishikori, a 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 percent of his first serves to 48 percent for Becker.
Nishikori, who scored three points to lead Japan over Canada in a Davis Cup tie earlier this month, reached a last-eight matchup Friday against Russian Alex Bogomolov, who defeated American Ryan Harrison 6-3, 2-6, 6-2 in another second-round match 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 Tuesday matchup 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.
Results on Wednesday from the $647 000 ATP US Indoor Championship (x denotes seeding):
Michael Russell (USA) bt Tim Smyczek (USA) 6-2, 7-6 (7/5)
Jack Sock (USA) bt Jiri Vesely (CZE) 6-2, 6-1
Alex Bogomolov (RUS) bt Ryan Harrison (USA) 6-3, 2-6, 6-2
Kei Nishikori (JPN x1) bt Benjamin Becker (GER) 6-4, 6-4
Denis Kudla (USA) bt Donald Young (USA) 6-1, 6-1