The defending champion, who is the tournament's top seed, recovered from a difficult opening set to complete a 2-5-hour comeback victory at the Ariake Coliseum on Monday.
Alcaraz recovers after losing first set
Alcaraz made a strong start but struggled badly towards the end of the opening set, allowing Munar to turn the match around and take it 7-5.
The Spaniard then endured an extraordinary run in which he lost 16 consecutive points spanning the end of the first set and the beginning of the second.
However, Alcaraz quickly regained control. He increased his aggression from the baseline, applied greater pressure on Munar's serve and eventually levelled the match by taking the second set 6-3.
The 23-year-old then dominated the deciding set, using powerful serves, aggressive returns, forehands and well-timed drop shots to pull away from his compatriot.
He won the final set 6-1 to complete the comeback after two hours and 31 minutes.
Alcaraz praises his ability to fight back
Reflecting on his difficult start, Alcaraz admitted he was unsure what had happened during the first set.
He said his ability to remain in the contest and respond after losing the opening set was something he was particularly pleased with.
Alcaraz also acknowledged the quality of Munar's performance, noting that the conditions suited his opponent and that he had to remain patient before increasing his aggression.
Lehecka awaits in the final
Alcaraz will face Czech Republic's Jiri Lehecka in Tuesday's final after the 19th-ranked player defeated eighth seed Valentin Vacherot 6-4, 6-2 in the other semi-final.
Lehecka reached the final without dropping a set in the tournament's semi-final and will be bidding for his first title of the season.
Alcaraz, meanwhile, will be attempting to successfully defend the Japan Open title and claim his first singles trophy since returning from a four-and-a-half-month absence caused by a right wrist injury.
A significant comeback for Alcaraz
The Tokyo run represents another positive step in Alcaraz's return to top form.
The Spaniard missed around four and a half months because of his wrist injury before returning at the US Open, where he reached the quarter-finals. He subsequently helped Team Europe win the Laver Cup before continuing his comeback in Tokyo.
His victory over Munar also extended his winning streak against fellow Spanish players to 16 consecutive matches since his defeat by Rafael Nadal at Indian Wells in 2022.
Match summary
• Carlos Alcaraz: defeated Jaume Munar 5-7, 6-3, 6-1
• Tournament: Japan Open, ATP 500
• Round: Semi-final
• Venue: Ariake Coliseum, Tokyo
• Duration: 2 hours, 31 minutes
• Final opponent: Jiri Lehecka
• Final: Tuesday, 6 October 2026