TOKYO — Second-seeded Gael Monfils overcame an ankle injury to beat Gilles Simon 6-1, 6-4 Thursday and advance to the quarterfinals at the Japan Open.

Monfils trailed 4-1 in the second set when he injured an ankle and had to call for the trainer. He won the next five games after receiving treatment.

“Somehow I got through my shots better when I came back, because maybe I was a bit worried,” Monfils said.

The Frenchman will face Ivo Karlovic in the quarterfinals. The big-serving Croat had 24 aces while beating Janko Tipsarevic 7-6 (9), 7-6 (5), and has now gone to a tiebreak in all five sets he has played in Tokyo.

“Last year I was losing a lot of tiebreaks. This year I’m a little more relaxed in them and trying not to panic,” the seventh-seeded Karlovic said. “My game plan today was that I would go to the net more often but it didn’t work out like that, so I had to be at the back a little more.”

Monfils beat Karlovic in Washington in July, the only time they have met since 2011.

“I was leading by a set and a break in Washington,” Karlovic said. “I was doing really well technically, hitting hard balls and he didn’t like it. Hopefully, I can also do that tomorrow and close out the match this time.”

Also, Gilles Muller of Luxembourg defeated Marcos Baghdatis 6-3, 6-4 to set up a match against Nick Kyrgios, who progressed when Radek Stepanek withdrew because of a back injury.

Top-seeded Kei Nishikori, who retired from his second-round match on Wednesday, pulled out of next week’s Shanghai Masters. He said he still hopes to be able to play at the season-ending ATP finals in London.

“The tests show I have a light strain in my left glute muscle,” Nishikori said in a statement. “The doctor is recommending at least 10 days’ rest and heavy rehab. I will do everything I can to get back on court and be ready for the end of the season.”