Unseeded American Jenson Brooksby has staged an improbable recovery to claim the biggest win of his career, sending world number three Casper Ruud packing in the second round of the Australian Open.

Brooksby looked a broken man — physically and mentally — after blowing three match points in the ninth game of the third set on Thursday.

The 22-year-old slumped to his chair during the changeover, muttering "How? How? How?" to himself and anyone else who cared to listen.

When Ruud claimed the resulting tiebreak, it seemed near-certain he would go on to seal victory.

But Brooksby somehow found the wherewithal to lift again, breaking the Norwegian's serve twice early in the fourth set before closing out the 6-3 7-5 6-7 (4-7) 6-2 upset on his fifth match point.

"Casper is a warrior and I knew it would be a great battle," said Brooksby, who is playing his maiden Australian Open, having missed the 2022 tournament due to a positive COVID-19 test.

"I'm really proud of my mental resolve after the third-set battle didn't go my way and to be able to turn it around."

His opponent in the third round will be fellow unseeded American Tommy Paul, who outlasted Spanish number 30 seed Alejandro Davidovich Fokina 6-2 2-6 6-7 (4-7) 6-3 6-4.

"That's going to be another battle for sure and hopefully we get a great court," Brooksby said.

Ruud's departure means the top two men's seeds have bowed out in the space of 24 hours after world number two and defending champion Rafael Nadal lost to another American, Mackenzie McDonald, on Wednesday.

Fifth-seeded Russian Andrey Rublev also booked his spot in the third round on Thursday with a convincing victory over Finland's Emil Ruusuvuori.

Rublev overcame a poor third-set tiebreak to win 6-2 6-4 6-7 (2-7) 6-3 in two hours, 42 minutes.