Second seed Casper Ruud follows Rafael Nadal out of Australian Open

A day after Rafael Nadal limped to defeat against another American, Mackenzie McDonald, Brooksby beat Norwegian Ruud 6-3, 7-5, 6-7 (4-7), 6-2 for the biggest victory of his career, recovering from missing three match points in the third set.

The result means the men's event has lost its top two seeds before the third round for the first time at any Grand Slam since 2002.

Ruud would certainly have had higher hopes for the tournament having reached two Grand Slam finals last year, at the French Open and US Open while he was also in with a chance to replace Carlos Alcaraz at the top of the ATP Rankings.

But he struggled from the start with the unorthodox style of 22-year-old Brooksby, who is named after former British racing driver Jenson Button.

He had reason to curse his luck, too, after losing the second set when a shot from Brooksby hit the tape and dribbled over the net.

Ruud then took an off-court medical timeout for an unspecified ailment and was clinging on by his fingernails at 5-2 down in the third but Brooksby was unable to serve it out, blinking at the finish line in agonising fashion.

Ruud looked like he might turn the match around when he took the tie-break, with Brooksby beginning to struggle physically, but he forged ahead again and finally clinched his fifth match point.

He raised weary arms in the air, and then said: "First and foremost, Casper is a warrior. I knew it would be great battle out there. I was pretty confident with my level and just wanted to have fun competing out there.

"I was just really proud of my mental resolve there, after the third-set battle didn't go my way, to just turn it around."

It has been a hugely successful tournament so far for the American men, and Brooksby was joined in the third round by Tommy Paul, JJ Wolf - who upset seeds in Alejandro Davidovich Fokina and Diego Schwartzman, respectively - and 20-year-old Ben Shelton, who is on his first ever trip outside the US.

Brooksby and Paul, who booked his third-round place with a 6-2, 2-6, 7-6, 6-7 (4-7), 6-3, 6-4 win over 30th seed Alejandro Davidovich Fokina, will meet next.

"That's going to be another battle for sure," Brooksby said. "A lot of Americans are doing really well right now and we're all pushing each other. I'm just looking forward to the next one."

Fifth seed Andrey Rublev also advanced, defeating Emil Ruusuvuori 6-2, 6-4, 6-7 (2-7) 6-3 to set up a third-round clash with Dan Evans.