Australian Open: Novak Djokovic ousted; American Tennys Sandgren topples No. 5 Dominic Thiem

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

The back-to-back upsets Monday have set up a very unexpected quarterfinal: Chung, the first South 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 (4), 7-5, 7-6 (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 3 1/2-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.”

The 26-year-old Sandgren, who entered the season’s opening major ranked 97th, 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 3-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 quarterfinals 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.

“Trying to keep riding the wave,” said Sandgren, who was named after his great-grandfather and who comes from Tennessee.

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

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

Angelique Kerber, the only Grand Slam singles winner remaining in the women’s draw, got a serious wake-up call.

For a while it appeared Kerber’s progression could unravel against No. 88 Hsieh Su-wei, a former top-ranked doubles player with a double-handed grip on both sides, until she regained momentum for a 4-6, 7-5, 6-1 win.

That earned Kerber a quarterfinal spot against U.S. Open finalist Madison Keys.

Keys returned to the quarterfinals here for the first time in three years with a 6-3, 6-2 win over No. 8-seeded Caroline Garcia, and is yet to drop a set so far.

Top-seeded Simona Halep, who had to rally from triple match point down to advance through the third round, beat Naomi Osaka 6-3, 6-2.