Alex de Minaur has missed out on a top-four US Open seeding after crashing out in the fourth round of the Cincinnati Masters.

De Minaur had the chance to climb into the world's top five with a deep run at the final Masters 1000 event before the season's last grand slam in New York.

But the Australian number one delivered a flat performance to fall 6-3, 6-4 to rising Frenchman Arthur Fils in just 82 minutes on Wednesday (Thursday AEST).

De Minaur dropped serve once in each set and was unable to break the 21st seed at all.

The world number eight's exit means de Minaur is certain to be seeded in the top eight at Flushing Meadows, still ensuring the Australian does not run into a higher-ranked player until at least the quarterfinals.

But with Carlos Alcaraz still injured, de Minaur could have snatched a US Open top-four seeding and been guaranteed to avoid heavyweights such as Jannik Sinner and Alexander Zverev until at least the semifinals.

Australian qualifier Chris O'Connell plays his fourth-round match in Cincinnati later on Thursday against American sixth seed Taylor Fritz.

AAP