Stefanos Tsitsipas' woes continued after the Greek No. 4 seed exited the Japan Open with a 4-6, 6-1, 6-2 first-round loss to American Alex Michelsen on Thursday.
Tsitsipas, the world No. 12, took the first set in Tokyo but had no answer to his 49th-ranked opponent for the rest of the match.
Tsitsipas, a two-time Grand Slam finalist, also lost in the first round of the U.S. Open last month.
He announced last month that he was ending his long-time coaching collaboration with his father Apostolos.
After losing to Australian Thanasi Kokkinakis at the U.S. Open, Tsitsipas said that he was "nothing compared to the player I was before," speaking of "longterm burnout."
The former world No. 3 has slipped out of the Top 10 and has won just two titles in two seasons.
Michelsen was joined in the second round by Britain's U.S. Open semifinalist Jack Draper, who beat Italian qualifier Mattia Bellucci 6-4, 6-2.
American Tommy Paul, the fifth seed, beat Italy's Matteo Arnaldi 6-3, 6-2.
Yoshihito Nishioka edged Canada's Felix Auger-Aliassime 7-6 (7-5), 3-6, 7-6 (7-5) in a match that lasted 3 hours, 12 minutes.
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