US Open champion Alcaraz crumpled to the ground chasing a shot in the fifth game of the opening set in Tokyo and sat on the court grimacing for several minutes.

The Spaniard had his left ankle heavily strapped during a medical timeout before gingerly returning.

He soon began to move comfortably and broke world number 41 Baez to take a 5-4 lead.

The match was delayed for a further half an hour as organisers closed the stadium roof with rain falling.

Alcaraz returned strongly and closed out the first set before breezing past Baez in the second.

"I have to see later if the outcome is going to be good," Alcaraz said of his injury after the match.

Alcaraz is playing at the Japan Open for the first time and the crowd gave him a warm cheer as he emerged in a burgundy sleeveless outfit with platinum blond hair.

But their applause soon turned to concern when he went down injured halfway through the first set, with his reaction suggesting he may be unable to continue.

Alcaraz somehow found a way to carry on and went on to dominate the match, winning in 1 hour and 30 minutes when Baez hit a return long.

Number two seed Taylor Fritz of the US survived a tough opening test against Canada's Gabriel Diallo to go through 4-6, 6-3, 7-6 (7/3).

Fritz, the world number five, beat Alcaraz at the Laver Cup in San Francisco last week.

Norway's number four seed Casper Ruud also advanced, with a 4-6, 6-1, 6-1 win over Japan's Shintaro Mochizuki.

Number five seed Tomas Machac of the Czech Republic went out after losing 6-3, 7-6 (7/4) to Japanese qualifier Sho Shimabukuro.