LOS ANGELES (AFP) - Three-time defending champion Kei Nishikori and Japanese compatriot Yoshihito Nishioka, a 130th-ranked qualifier, advanced to the quarter-finals of the ATP Memphis Open on Wednesday with straight-sets triumphs.

Top seed Nishikori defeated 124th-ranked Ryan Harrison 6-2, 7-5 to book a last-eight date against Kazakstan's Mikhail Kukushkin in the indoor hard-court tournament.

Seventh-ranked Nishikori beat the American in the second round at Memphis for the second year in a row. The Japanese also won their only other prior meeting last year at Indian Wells.

Nishikori served for the match in the 10th game of the second set but Harrison denied him on four match points and forced the first break points the Asian star faced in the match, finally cracking his serve on his fifth chance to level the set at 5-5.

The Japanese responded by breaking Harrison again in the next game and holding at love to claim the victory after 81 minutes.

Left-hander Nishioka downed Ukraine's Illya Marchenko 6-1, 6-3 to book a quarter-final match against US fourth seed Sam Querrey, who eliminated Swiss qualifier Henri Laaksonen 6-3, 6-4.

Kukushkin advanced by ousting US fifth seed Denis Kudla 6-1, 7-5, winning 22 of 30 points off the American's second serve.