Nick Kyrgios will face Luxembourg’s Gilles Muller in the Japan Open quarter-finals, with the Australian being gifted a run into the final eight.

The tournament’s sixth seed, Kyrgios moved through to the quarters without hitting a ball when his opponent Czech veteran Radek Stepanek withdrew with a back injury.

World No.106 Stepanek, who turns 38 next month, failed to take to the court.

He has been acting as a part-time mentor for 21-year-old Kyrgios, who has risen to 15th in the world rankings.

Kyrgios has won two titles this season, in Marseille and Atlanta, but faces an in-form player in Muller, the world No.36 who is just two spots shy of his highest ranking.

The 33-year-old overcame Cypriot Marcos Baghdatis 6-3 6-4 in their second-round clash.

Marin Cilic, David Goffin, Juan Monaco and Joao Sousa have advanced to the quarters, with top seed Kei Nishikori pulling out with a back injury midway through Wednesday’s match with Sousa.

Second-seeded Gael Monfils overcame an injury scare and beat French compatriot Gilles Simon on Thursday to also secure a last-eight berth.

Monfils swept through the first set 6-1 but was trailing 4-1 in the second when he injured an ankle and had to call for the trainer.

Having received treatment, he returned to dominate the rest of the set and clinch a 6-1 6-4 victory.

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

He will meet Ivo Karlovic or Janko Tipsarevic in the quarters.