Unheralded American Tennys Sandgren says he struggling to come to grips with upsetting Austria’s fifth seed Dominic Thiem over five sets to storm into the quarter-finals of the Australian Open on Monday.

The 97th-ranked Sandgren won 6-2, 4-6, 7-6 (7/4), 6-7 (7/9), 6-3 in just under four hours to continue his extraordinary run and will face South Korean giantkiller Chung Hyeon in the last eight.

He becomes just the second man in the last 20 years since Frenchman Nicolas Escude to make the quarter-finals on his Australian Open debut.

Amazingly, the 26-year-old missed out on qualifying in the last four years to reach the main draw in Melbourne.

Sandgren conquered former winner Stan Wawrinka in the second round and this time eliminated the world No 5.

“The first three matches were more than I expected. This one was about as hard-fought as I’ve ever had a match before,” Sandgren said.

“My biggest match, as well, pretty neat. I definitely had a real pinch-me moment. Wow, this is hopefully real. If I wake up now, I’m going to be real upset!”

The man from Tennessee, who hadn’t won a Grand Slam match until this tournament after being knocked out in the first round at last year’s French and US Opens, said the wins had given him belief.

“I know that I’m good enough to do good things in the game. This is confirmation for me,” he said.

“I know that I serve well. I know I can take care of business on my serve. If you can hold serve in this game, you can compete with anybody.

“My movement is good. Playing good defence. Most importantly I’m staying calm and not getting too upset, able to keep my emotions under control.”

Sandgren said he had the advantage of being an unknown player in the field.

“Maybe guys aren’t sure what to expect and they don’t know that I’m serving well and what spots I like, or how I’m going about playing the points, and I’m using that to my advantage,” he said.

A tremendous backhand winner down the line from Thiem saved a match point and then he went on to force a fifth set, but Sandgren finished strongly to take the match.

Sandgren hit 63 winners, 20 aces and made four service breaks to send the highly-rated Thiem tumbling out of the tournament.