Cincinnati - Kei Nishikori lost to fellow Japanese Yoshihito Nishioka 7-6 (7/2), 6-4 on Wednesday in the second round of the ATP Cincinnati Masters.
The sixth seed went down to his 77th-ranked compatriot, who came through qualifying, in just over 90 minutes.
"It was a tough match for me. I think he played good tennis, served well," Nishikori said.
"I was happy to play him for the first time. It's good to see he's getting stronger, growing up. He was already good, he gets a lot of balls back and has good defence."
Nishikori has never advanced beyond the third round at the US Open warm-up event.
Nishioka will next face Australia's Alex de Minaur, who was a 7-6 (7/3), 6-4 winner over American wildcard Reilly Opelka.
Results on Wednesday in the WTA/ATP Cincinnati Masters (x-denotes seeding):
Richard Gasquet (FRA) bt Federico Delbonis (ARG) 7-5, 7-6 (7/1)
Yoshihito Nishioka (JPN) bt Kei Nishikori (JPN x6) 7-6 (7/2), 6-4
Alex De Minaur (AUS) bt Reilly Opelka (USA) 7-6 (7/3), 6-4
Aryna Sabalenka (BLR x9) bt Zheng Saisai (CHN) 6-4, 6-3
Karolina Pliskova (CZE x3) bt Wang Yafan (CHN) 6-1, 6-3
Rebecca Peterson (SWE) bt Veronika Kudermetova (RUS) 2-6, 7-5, 6-2
Donna Vekic (CRO) bt Victoria Azarenka (BLR) 6-2, 7-5
Simona Halep (ROM x4) bt Ekaterina Alexandrova (RUS) 3-6, 7-5, 6-4
Elina Svitolina (UKR x7) bt Elise Mertens (BEL) 6-4, 6-1