Giant-killing Alex Michelsen can scarcely believe he's reached the Australian Open fourth round after pulling off a Pete Sampras-like feat to power into the second week of a grand slam for the first time.

The 20-year-old Michelsen followed up his shock first-round removal of world No.11 and 2023 Open runner-up Stefanos Tsistipas with a stunning straight-sets defeat of 19th seed Karen Khachanov on Saturday.

In claiming the two big scalps, Michelsen is the youngest American since Sampras and four-time AO champion Andre Agassi at the 1990 US Open to record multiple top-20 wins at a grand slam.

"I played unbelievable in that match. I don't even really know what's going on," Michelsen said after seeing off Khachanov 6-3 7-6 (7-5) 6-2 in two hours and 14 minutes.

"I was hitting my forehand unbelievable. I've never hit my forehand that well, so super happy to keep it going in the next round and, yeah, not a lot of words right now."

Khachanov was a semi-finalist at Melbourne Park two years and shaped as a road block between Alex de Minaur and a place in the quarter-finals for the first time for the eighth-seeded home hope.

Instead Michelsen will be standing in de Minaur's way after the Australian eighth seed survived a third-round scare to care of Argentina's world No.31 Francisco Cerundolo 5-7 7-6 (7-3) 6-3 6-3.

While Michelsen marched on, fourth-seeded countryman Taylor Fritz was eliminated by French veteran Gael Monfils, who continued to wind back the clock with a 3-6 7-5 7-6 (7-1) 6-4 boilover.

The 38-year-old extended his hot run of form, taking his record to 9-1 for the season with an eighth straight victory after winning the Auckland title earlier this month.

A two-time Open quarter-finalist in 2016 and 2022, Monfils can match his previous best result if he beats either Ben Shelton or Lorenzo Musetti in the fourth round.