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.
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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.
