SIX-time champion Novak Djokovic was stunned in straight sets by Hyeon Chung only hours after Tennys Sandgren beat No 5 Dominic Thiem at the Australian Open.

The back-to-back upsets today have set up a very unexpected quarter-final: Chung, the first Korean to reach the last eight at a grand slam, versus 97th-ranked Sandgren, who had never won a match at a major or beaten a top-10 player until last week.

No 58-ranked Chung relentlessly attacked Djokovic — who is playing his first tournament since Wimbledon last July because of an injured right elbow — in the 7-6 (7/4) 7-5 7-6 (7/3) fourth-round win.

He ripped 47 winners, including a forehand on the slide and at full stretch that put him within two points of victory, in the almost three-and-a-half-hour match.

Chung credited the usually athletic Djokovic, who needed a medical timeout in the second set for a massage on his sore elbow, for the inspiration for that unlikely shot.

“When I’m young, I’m just trying to copy Novak because he’s my idol,” Chung said. “I can’t believe this tonight. Dreams come true tonight.”

Sandgren missed a match point in the fourth set but held on for a 6-2 4-6 7-6 (4) 6-7 (7) 6-3 win over Thiem. It followed up his earlier victory over 2014 champion Stan Wawrinka.

“I don’t know if this is a dream or not — all you guys are here, so maybe it’s not,” he said in an on-court TV interview after his three-hour, 54-minute fourth-round win. “I’m not in my underwear, so maybe it’s not a dream.”

Sandgren is only the second man in 20 years to reach the quarter-finals on his debut at Melbourne Park.

He converted half of his eight break-point chances, and fended off 10 of the 12 he faced against Thiem, and hit 63 winners against 38 unforced errors in the biggest win of his life.

Sandgren said he’s “trying to keep riding the wave.”

Defending champion Roger Federer had no real difficulties in reaching the quarters for the 14th time, accounting for Marton Fucsovics 6-4 7-6 (3) 6-2.

The 19-time major winner had never played Fucsovics but had beaten his coach — Attila Savolt — here in the second round in 2002.

Federer will renew a lengthy rivalry next against Tomas Berdych, who returned to the quarter-finals for the seventh time at Melbourne Park with a 6-1 6-4 6-4 win over Fabio Fognini.