Carlos Alcaraz began his defence of the Japan Open title with a battling 7-6 (7/1), 4-6, 6-1 win over Alex Michelsen in Tokyo on Thursday.
The Spanish world number three struggled to find his rhythm early on, spraying shots long or into the net as the big-serving American stayed aggressive.
He pulled away in the tie-break but was pinned back in the second set, with Michelsen, ranked 34th, gaining parity after a marathon 10th game.
Going into the third set, though, the seven-time Grand Slam champion suddenly found form, mixing a barrage of blistering aces with delicate volleys to nullify his opponent's threat.
It was ultimately a heartening performance for Alcaraz, who only returned to action at the US Open in August after a four-month injury layoff.
He said it took time to adjust to the open-roofed main court after practicing indoors earlier in the day.
"Going on the central court, I felt a totally different game, different conditions, different everything. So I just tried to play my best," he said.
"I'm happy to get the win at the end but I'm a little bit unhappy about my level."
Alcaraz will play Italy's Matteo Arnaldi, ranked 37th, in the next round.
Earlier, second-seeded Frances Tiafoe cruised past Kei Nishikori 6-4, 6-4 in the beloved home star's final career match.
Nishikori, Asia's most decorated men's tennis player, said in May he was calling time on a career that might have taken him to the pinnacle of the game were it not for persistent injury.
"I am happy that in my spirit I remained competitive until the end," he said in an emotional post-match ceremony.
In other matches, Spanish qualifier Jaume Munar upset third-seeded Taylor Fritz, the 2022 champion, 7-6 (9/7), 4-6, 6-3.
Sixth seed Casper Ruud also crashed out to Luciano Darderi of Italy, 7-6 (7/3), 6-3.
And fifth-seeded American Brandon Nakashima also left the tournament with a 6-3, 6-2 defeat to Frenchman Ugo Humbert.