The world number three clinched the win despite struggling with mistakes, particularly near the end of the first set, while his 49th-ranked opponent put up a brave fight.
"Right now, I'm trying to get my level back. I would say it's still far from it, still far from it," he said when asked about his current condition compared with his success last year.
He said he was "feeling good" but was still "trying to get into the competition, trying to get into the rhythm of playing matches, playing tournaments".
The Australian Open champion clinched the Tokyo win despite struggling with mistakes, particularly from the end of the first set through the middle of the second set, when his 49th-ranked opponent put up a brave fight.
Alcaraz unleashed a barrage of big serves and aggressive returns to keep Munar off-balance and running along the baseline, occasionally mixing in delicate drop shots that wowed the packed crowd at the Ariake Colosseum.
Munar maintained his composure to stay competitive, especially late in the opening set, as Alcaraz hit shots wide and long and racked up four double faults in that set alone.
Nevertheless, Alcaraz kept pressure on his opponent, showering him with precise forehands, fast volleys and drop shots, breaking Munar's serve in the first, third and ninth games of the second set.
Alcaraz dominated the final set, wrapping up the match in two hours and 31 minutes.
"I don't know what happened until the middle of the second set," Alcaraz said.
"Last year I came here with a lot of trophies... with a lot of confidence. But I'm going to fight for the same result, which I'm really happy about," he said.
In the final, Alcaraz will face 19th-ranked Jiri Lehecka of Czechia, who beat Valentin Vacherot of Monaco 6-4, 6-2.