Tennys Sandgren said he was not sure if he was dreaming after he continued his remarkable run at the Australian Open by eliminating fifth seed Dominic Thiem to reach the quarter-finals.

Finally playing in the main draw at Melbourne Park after repeated failed attempts to make it through qualifying, world number 97 Sandgren shocked Stan Wawrinka in round two and claimed another notable scalp on Monday.

Despite seeing a match point go begging in a dramatic fourth-set tie-break, the American prevailed 6-2 4-6 7-6 (7-4) 6-7 (7-9) 6-3 to become the first player since Nicolas Escude in 1998 to make the last eight of the Australian Open on his main-draw debut.

"I don't know if this is a dream or not. All you guys are here, so maybe it's not ... and I'm not in my underwear, so maybe it's not a dream," said Sandgren in an on-court interview following his stunning win.

The 26-year-old shook his head in disbelief as he added: "This is my fifth Australian Open, I've lost in qualifiers four years and never made it into the main draw. And now I'm in the quarter-finals."

Sandgren, who will now face Hyeon Chung in the quarter-finals, feels he could be benefiting from being something of an unknown quantity given his lack of experience on the big stage. This is only his third appearance in the main draw of a grand slam after he was knocked out in the first round of last year's French Open and US Open.

"Maybe guys aren't sure what to expect and they don't know that I'm serving well or what spots I like or how I go about playing points," he explained.

"I am using that to my advantage and just trying to keep riding the wave."