Alcaraz saved three ‌set points before clinching a see-sawing opening set ‌with a strong display in the tiebreak but ‌continued to experiment with his new sped-up service motion and was broken twice early in the next ‌set.

The 23-year-old Spaniard, who had shown ⁠glimpses of his ‌best form en route to the US Open quarter-finals last ​month after his long injury layoff, recovered to move ahead but a double ​fault in the seventh game handed Michelsen a route back.

Michelsen capitalised and kept his composure in ⁠a tense ​finish to the second set where a fortunate netcord winner left Alcaraz stranded at the baseline and helped the 22-year-old American force a decider.

Alcaraz, who led Team Europe to a Laver Cup triumph over Team ​World on indoor courts in London last weekend, said the relentless rain in Tokyo limited his practice.

"It was a different feeling. The ball ⁠comes bigger. The court ​performs a little bit slower than the indoor courts for sure, so it was freaky to adjust to that," Alcaraz said.

"It was a little bit frustrating, not being able to play my game, my style, and play ‌aggressive ... it was difficult but I'm really happy to accept and understand how I had to play and to get the win at the end.

"It was really important to have this feeling for the next round, to know what I have to do for the next matches."

A couple ‌of breathtaking points gave Alcaraz a 2-0 lead in the final set and the seven-time Grand Slam champion used the launch pad to close out a comfortable victory on a day of big upsets in the ATP 500 tournament.

The top seed's win came after third seed Taylor Fritz fell to qualifier Jaume Munar 6-7 (7), ‌6-4, 6-3, sixth seed Casper Ruud lost ​to Luciano Darderi 7-6 (3), 6-3 and seventh seed ‌Tommy Paul suffered a 6-1, 6-2 defeat by Alejandro Tabilo.

Holger Rune, the former world number four who is stepping up his comeback from an Achilles injury, also crashed out ⁠following a 6-3, ⁠6-1 loss to Kyrian ‌Jacquet.