A tired-looking Dominic Thiem became the highest seed to fall at this year’s Australian Open when the US Open champion was sent tumbling out in straight sets by Grigor Dimitrov in the fourth round Sunday while ailing world No 1 Novak Djokovic battled past Milos Raonic to keep his title defence alive.

The night match’s scheduling gave Djokovic the maximum amount of time to recover from a midriff muscle injury sustained in his five-set win over American Taylor Fritz late on Friday. And not operating at his best, Djokovic dropped a set but won 7-6(4), 4-6, 6-1, 6-4 in a bruising battle. It was the Serb’s win No 300 in Grand Slam events.

The 17-time Grand Slam winner said he had spent “every single hour” since Friday’s win trying to recover, and wasn’t sure he would play until he warmed up before the match.

“If it was any other tournament than a Grand Slam I would retire from the tournament, that’s for sure,” said Djokovic, who becomes just the second player after Roger Federer to win 300 Grand Slam matches.

“But because it’s a Grand Slam, I want to give my best alongside my team to try to recover and get on the court.”

Earlier, Thiem, the Austrian third seed and last year’s losing finalist, appeared to lack energy after rallying to beat Nick Kyrgios over five gruelling sets on Friday in front of a raucous crowd. He struggled to match the intensity of the 18th-seeded Bulgarian on a silent, fan-free Rod Laver Arena and slumped out 6-4, 6-4, 6-0 in 2hr 1min.

Former world No 3 Dimitrov will now play giant-killing Russian qualifier Aslan Karatsev in the quarter-finals after he stunned 20th seed Felix Auger-Aliassime over five sets. The power-hitting 27-year-old, a virtual unknown before the tournament started, battled back from two sets down to upset the Canadian 3-6, 1-6, 6-3, 6-3, 6-4 on Margaret Court Arena.

Australian Open: The numbers from debutant qualifier Aslan Karatsev’s run to the quarter-finals

Dimitrov, who has yet to drop a set, has now reached the quarter-finals or better in Melbourne four times, with his best showing in 2017 when he lost an epic five-set semi-final to Rafael Nadal.

Thiem looked to be in charge when he raced into a 3-1 lead, but Dimitrov came storming back in an energy-sapping 14-minute game to break back with a backhand passing shot. He broke again in the seventh game to take the set with Thiem unable to make any impression against his opponent’s powerful and accurate serving.

Dimitrov had break point opportunities in the opening game of the second set, but Thiem held on and went 3-1 in front before the unrelenting Bulgarian once again levelled. He was in the zone and broke again then served out the set to love and with a now error-strewn Thiem struggling, steamrollered through the third set in just 21 minutes.

Next up for Dimitrov is giant killer Karatsev, the he Russian qualifier who became the first player to reach a Grand Slam quarter-final on debut in 25 years, stunning 20th seed Auger-Aliassime. It continued his giant-killing run after blitzing eighth seed Diego Schwartzmann in round three.

The world No 114 is the first Grand Slam debutant to reach the quarter-finals at a major since Romanian Alex Radulescu at Wimbledon in 1996 and only the seventh in the Open era.

He is the first qualifier to achieve the feat since Australia’s Bernard Tomic at Wimbledon in 2011 and only the third ever at the Australian Open after Bob Giltinan in 1977 and Goran Ivanisevic in 1989.

The bearded Karatsev has been on the verge of playing a Grand Slam for years, but on nine previous attempts the injury-dogged Russian fell in qualifying.

He was part of the victorious Russian ATP Cup team this month along with Daniil Medvedev and Andrey Rublev, who play their fourth-round matches on Monday.

Later, Alexander Zverev totted up his 50th Grand Slam win when he beat Serbia’s Dusan Lajovic in straight sets to reach the quarter-finals.

The German sixth seed was tested in the first two sets but found it easier in the third, sealing his 6-4, 7-6 (7/5), 6-3 victory with an ace.

Zverev avoided his usual tussle with the 27th-ranked Lajovic, after needing five sets in both their previous meetings at the 2018 and 2019 French Opens.

The 23-year-old has dropped only one set en route to the last eight, where he will face defending champion Djokovic.

Last year’s US Open runner-up is into the Australian quarter-finals for the second straight year as he targets his maiden Grand Slam title.

With AFP Inputs

Men’s singles fourth round:

Novak Djokovic (SRB x1) bt Milos Raonic (CAN x14) 7-6 (7/4), 4-6, 6-1, 6-4

Alexander Zverev (GER x6) bt Dusan Lajovic (SRB x23) 6-4, 7-6 (7/5), 6-3

Grigor Dimitrov (BUL x18) bt Dominic Thiem (AUT x3) 6-4, 6-4, 6-0

Aslan Karatsev (RUS) bt Felix Auger-Aliassime (CAN x20) 3-6, 1-6, 6-3, 6-3, 6-4