The world number three said his body was still adjusting to playing demanding matches after coming back from a wrist injury that sidelined him from April until the US Open in August.
But he comes into Shanghai having won the Japan Open on Tuesday, beating Jiri Lehecka 7-6(4), 6-4 in the final.
"I'm feeling very good. Physically I'm well, I'm feeling mentally fresh," the 23-year-old told reporters in the Chinese city.
"I'm training very well, so we'll see."
Alcaraz has never gone beyond the quarter-finals in Shanghai, and missed last year's tournament due to fitness issues.
The former world number one has previously criticised the packed tennis schedule, particularly the autumn "Asian swing" that follows the US Open.
"If we see a couple years back, you know, probably this swing or this part of the season, it's never been my best one," Alcaraz said.
But he added that he was "ready to take on the challenge" in Shanghai, where he will kick off his campaign against either Argentina's Juan Manuel Cerundolo or qualifier Nicolas Mejia of Colombia in the second round.
Alcaraz's path to victory has opened up after current world number one Jannik Sinner announced on Tuesday he was ending his 2026 season with a knee injury.
His main rivals in the draw are Germany's Alexander Zverev, the current world number two, and Serbian veteran Novak Djokovic, who claimed his 102nd career title in Beijing on Tuesday.
Djokovic hails Sinner's 'big-picture' thinking
Djokovic, 39, told reporters in Shanghai that Sinner was "smart" to call time on his season to heal.
"He has to think long-term," Djokovic said. "That's smart because, you know, as someone that... always tried to have the big picture in my mind, I really support that."
"It's important that you give yourself more years to your career, so you have more chances to accomplish all of your objectives," added the 24-time Grand Slam champion.
Djokovic is riding the momentum of his Beijing triumph into the prestigious Shanghai event, which he has won a record four times but not since 2018.
"It was a phenomenal week for me, Beijing, the best week of this year, no doubt," the former world number one said.
"I'm trying to... use the confidence and hopefully have another great deep run."
Djokovic will start his tournament against Poland's Hubert Hurkacz in the second round.