Alexander Zverev was forced to dig deep in beating Quentin Halys in three sets at the Shanghai Masters on Sunday as he closes in on the world No 1 ranking.
The German top seed belatedly saw off the Frenchman 6-4, 6-7 (8/10), 6-3 to reach the last 16, despite moving a little uncomfortably at the end.
"He plays unbelievably aggressive and serves incredibly," Zverev, himself one of the biggest servers on tour, said of his beaten opponent.
"This year I have been picking my tournaments and places where I want to peak, I did not do that early in my career.
"It paid off, I won two Grand Slams and reached the Wimbledon final."
Zverev cut a frustrated figure at times, barking at his corner even as he clinched the first set against the 48th-ranked Halys in 38 minutes.
The second set was much closer, going with serve and taking them into the tie-break.
Zverev unleashed a delicious backhand down the line to avoid going down 6-4 in the tie-break, but then squandered a match point when he shanked a forehand return.
A second match point came and went, and Halys took full advantage to force a deciding set courtesy of an overhead smash.
The third was poised for another battle, but Zverev broke for 2-0 and behind his reliable serve powered home for victory.
Zverev, the world No 2, is an imminent threat to Jannik Sinner's No 1 ranking.
The US Open and French Open winner has never topped the rankings.
The Italian Sinner is not playing in Shanghai because of injury, opening the door for the 29-year-old Zverev to usurp him.
World No 3 Carlos Alcaraz is in action on Monday.
The Spaniard faces Belgium's Zizou Bergs in the third round.