It was the first ATP win on hard courts for the 25-year-old Hijikata, who played collegiately at North Carolina. He will face Italian Luciano Darderi in the second round.
Monfils made his 14th and final appearance at the Cincinnati Open. The 39-year-old Frenchman has announced he's retiring at the end of 2026.
Monfils reached the Cincinnati quarter-finals in 2011 and upset third-ranked Carlos Alcaraz in the second round in 2024. He received a standing ovation from the crowd following Thursday's match (Friday AEST).
Another Australian, Aleksandar Vukic lost in the second round of qualifying, to Estonia's Mark Lajal 7-6 3-6 3-4.
In the main draw, Jack Draper fell 6-3 7-5 to Martín Landaluce in their first-ever meeting. Landaluce, who snapped a six-match losing streak, will face Matteo Arnaldi in the second round.
Draper, who achieved a career-high No.4 ranking in June 2025, was playing in just his third tournament since May due to an arm injury.
Cameron Norrie rallied to beat Dino Prizmic, 3-6 6-1 6-4 to earn a date with top seed and third-ranked Alexander Zverev in the second round.
The Cincinnati Open lost the world's top-ranked players this week when Jannik Sinner and Carlos Alcaraz withdrew due to injury.
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