Carlos Alcaraz admitted his ankle injury had worried him and was on his mind as he beat Belgium's Zizou Bergs 6-4, 6-3 at the Japan Open on Saturday.
The world No 1 hurt himself two days earlier in his opening match in Tokyo, crumpling to the ground before playing on with his ankle heavily strapped.
The Spaniard was left sweating on his fitness but he showed no evidence of discomfort as he returned to the court to beat world number 45 Bergs in front of an adoring Japanese crowd.
Alcaraz got the job done in an hour and 20 minutes and will face American Brandon Nakashima in the quarterfinals on Sunday.
"I was a bit worried before the match so I just wanted to warm up and let's see how it's going to feel," Alcaraz said.
"I didn't feel anything in the ankle, so after the warm-up I decided I can go and I can play."
Alcaraz took to the court with white strapping visible underneath the sock on his left ankle.
He did not appear to be restricted in his movement but he limped briefly after chasing a shot that won Bergs a break early in the first set.
"In some movements I'm scared about going crazy so most of the time I take it a little easy," he said.
"Sometimes I feel a little bit the ankle, which I would say is normal."
Alcaraz is playing in Japan for the first time and delighted the crowd with a series of leaping forehand winners, claiming the first set despite having his service broken twice.
His service was broken three times overall and he blamed his injury for his lack of accuracy.
"I think because my thoughts were on the ankle, I forgot about doing a smooth movement, smooth serve," he said.
Alcaraz raced into an early lead in the second set but Bergs made him work hard for the victory with a gritty performance.
Alcaraz sealed the deal with a thumping forehand at the net that Bergs could not return into play.
Alcaraz will look to book his place in the semifinal when he faces world number 33 Nakashima, who beat Hungarian qualifier Marton Fucsovics 7-5, 6-3.
"I'm still taking care of the ankle and I will try to feel as good as today or even better," said Alcaraz.
"Tomorrow is going be a great challenge that I'm really excited about, and I'm ready for that."
Denmark's No 3 seed Holger Rune also advanced, beating American qualifier Ethan Quinn 6-4, 6-2.