Nishikori into Memphis last eight

Memphis - Two-time defending champion Kei Nishikori rallied to get past US qualifier Ryan Harrison on Wednesday to book his quarter-final spot at the ATP Memphis Open.

The Japanese top seed, who reached the quarter-finals of the Australian Open in January, gained a key early break in the third set and finally secured the 3-6, 6-3, 6-4 win on his fourth match point.

"It was a tight game, but I'm really happy to win," the 25-year-old said. "I think it's important to win something like this sometimes, it's good for confidence."

Nishikori, who enjoyed a first-round bye before opening his title defense against Harrison, said he feels like Memphis is "becoming my tournament."

"I know it's not easy to defend titles, but I think mentally I'm ready," he said.

Nevertheless, Nishikori said he'll have to play better in the quarter-finals, where he'll face another American qualifier, Austin Krajicek.

Krajicek, who survived two match points in a first-round win over Mikhail Kukushkin, toppled fifth-seeded Croatian Ivo Karlovic 7-6 (7/4), 4-6, 6-4.

Krajicek withstood a barrage of 32 aces from last year's runner-up, but served 14 of his own.

"It was a great match," Krajicek said. "Obviously, Ivo has an amazing serve, so you don't have a whole lot of chances. I played a really good tie-break in the first set -- I tried to take what I could control, and I just got a little lucky there at the end."

Third-seeded American John Isner pounded 22 aces in a 6-4, 7-6 (7/5) victory over Croatian Ivan Dodig.

Isner next faces compatriot Sam Querrey, who needed only 57 minutes to get past Jared Donaldson 6-4, 6-1.

The second round continues on Thursday, when second-seeded wild card Kevin Anderson of South Africa headlines the action taking on Australian Sam Groth.

Fourth-seeded Ukrainian Alexandr Dolgopolov also faces a challenger from Down Under in Bernard Tomic.

2nd rd

Kei Nishikori (JPN x1) bt Ryan Harrison (USA) 3-6, 6-3, 6-4

Austin Krajicek (USA) bt Ivo Karlovic (CRO x5) 7-6 (7/4), 4-6, 6-4

John Isner (USA x3) bt Ivan Dodig (CRO) 6-4, 7-6 (7/5)

Sam Querrey (USA) bt Jared Donaldson (USA) 6-4, 6-1