Top-seeded Alexander Zverev toughed out two tie-breaks to beat Alexander Bublik 7-6 (8/6), 4-6, 7-6 (7/1) at the Shanghai Masters on Tuesday, winning his first match since being guaranteed the year-end world number one ranking.
The 29-year-old German will play Alex de Minaur in the quarter-finals.
"Really difficult match," Zverev said.
"I had some physical issues so, you know, I was maybe slower," he told the crowd.
"My ankle was acting up unfortunately again," Zverev elaborated in an on-court interview, without giving specifics.
"I'm starting to feel my body more, but happy to be through."
Zverev, the current world number two, was guaranteed the year-end number one spot after third-ranked Carlos Alcaraz lost to Zizou Bergs in Shanghai on Monday.
On Tuesday, Zverev called the honour "special", saying it was a different feeling to winning tournaments, adding he was "looking forward" to the responsibility of owning that status.
Neither player could create much trouble for his opponent in the first set as they traded aces to hold serve.
