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Monfils was 3-1 up in a second-set tie-break before rain stopped play on Friday (AEST), but returned 24 hours later to win 4-6 7-6 (7-5) 6-4.

Once play resumed, Monfils continued where he left off but squandered three set points before eventually taking the tie to the distance.

Alcaraz, last year's runner-up, struggled in the opening exchanges, with Monfils finding a decisive break in the third game to take control of proceedings in Ohio.

The 37-year-old Frenchman's experience ultimately told, as he held out for the remainder of his service games to set up a meeting with Holger Rune in the Round of 16, condemning Alcaraz to a shock exit.

World No.1 Jannik Sinner, meanwhile, had no need to take to the court on his 23rd birthday, as he was handed a walkover after Australian Jordan Thompson's withdrawal.