Kei Nishikori opened his title defence with a straight-sets win over Ryan Harrison in the last 16 of the Memphis Open on Wednesday.
Defending champion Nishikori - bidding to win a fourth consecutive crown in Memphis - was a class above as he saw off Harrison 6-2 7-5 to advance to the quarter-finals.
Nishikori was making his first appearance since reaching the quarter-finals at last month's Australian Open, where he was knocked out by world number one and eventual winner Novak Djokovic.
But the Japanese star was relatively untroubled in the United States, despite serving at just 57 per cent on first serve, as he closed out proceedings after one hour, 21 minutes at the ATP 250 event.
Next up for the top seed is Kazakh Mikhail Kukushkin, who stunned fifth-seeded American Denis Kudla 6-1 7-5 to win through.
Sam Querrey had no such trouble in his run to the last eight.
The American fourth seed accounted for Swiss qualifier Henri Laaksonen 6-3 6-4.
Querrey's straight-sets win earned him a clash with Yoshihito Nishioka, who eased past Illya Marchenko 6-1 6-3.