In the evening session, Australia stunned Serbia to book the home team's spot in its first United Cup semi-final. World No. 12 Alex de Minaur led the way with an inspired upset over World No.1 Novak Djokovic 6-4, 6-4. De Minaur delivered a top-class all-around performance, during which he won 33 of 34 points behind first serves, to notch the biggest win of his career in 94 minutes.

Ajla Tomljanovic followed de Minaur to seal the victory, defeating Serbia's Natalija Stevanovic 6-1, 6-1 to give Australia an insurmountable 2-0 lead.

Stevanovic, ranked No. 184, was called on to step in for Olga Danilovic. Armed with a crafty slice-and-dice game, Stevanovic tested Tomljanovic's discipline and patience early. But the Australian was up to the task, especially after her confidence-boosting performance against No.5 Jessica Pegula on Tuesday. Ranked No. 292 after being sidelined by injury last year, Tomljanovic bounced back in November to notch an emotional title run at the WTA 125K in Florianopolis.

After breaking first for a 2-1 lead, Tomljanovic dominated the set to take it after just 31 minutes. She limited Stevanovic to just 14 points. The trend continued through the second set. Tomljanovic dominated with her forehand down the stretch to seal her first win at the United Cup and first win of the season.

Earlier, de Minaur laid down a stunning marker for his 2024 season — and Australia’s United Cup hopes — Wednesday at the United Cup in Perth.

The No. 12 in the Pepperstone ATP Rankings upset World No. 1 Novak Djokovic 6-4, 6-4 to give the host nation a 1-0 lead in their quarter-final against Serbia at the mixed teams event. De Minaur delivered a top-class all-around performance, during which he won 33 of 34 points behind first serves, to notch the biggest win of his career in 94 minutes.

"It's extra special,” De Minaur said of the win. “Novak is an unbelievable competitor and what he’s done for the sport is pretty special. It feels surreal, amazing, and I’m very happy to do it here in Perth and Australia.

"When you go up against Novak you have to go out and enjoy it and back yourself and no matter what keep fighting until the end. Today was my day and I’m happy that I was able to get the win."

With his victory against Djokovic, De Minaur moved his country within one win of a semi-final spot on home soil at the mixed teams event. His teammate Ajla Tomljanovic can book Australia’s ticket to the last four in Sydney by beating Natalija Stevanovic in the WTA singles match next inside RAC Arena.

“I’m here to prove a lot of people wrong and to try and get better," said De Minaur. "Ultimately I won’t be the biggest or the strongest. I will show I have variety in my game and I’m glad I could bring this level today.”

It was Djokovic’s first defeat in Australia since he fell to Hyeon Chung in the quarter-finals at the 2018 Australian Open. Serbia’s talisman was far below the level he showed in his season-opening victories against Zhang Zhizhen and Jiri Lehecka in Perth, and he again struggled with the right wrist issue for which he had a medical time-out against Lehecka on Tuesday.

Although he saved the first two break points of the match to hold for 4-3 in the opening set, Djokovic had treatment from the physio following that game and again after De Minaur had clinched an ultimately decisive break of serve at 4-5. He was unable to consistently hit through De Minaur, as the Australian showcased his electric defence to back up his dominant serving display.

De Minaur had won just five games across three sets in his only previous Lexus ATP Head2Head meeting with Djokovic at last year’s Australian Open. Yet the 24-year-old showed few nerves as he closed out a deserved win in Perth. The seven-time tour-level titlist broke again in the seventh game of the second set and his only lapse came when he let slip three match points from 5-4, 40/0 in the second set. He made no mistake with his fourth, however, as he put Australia on the brink of a spot in the last eight.

“Just really proud of the whole team,” team captain Lleyton Hewitt said. “Alex obviously came out and had to play the best player around. We knew that he had to go out there and play well and execute his game plan. He was able to do that from the start… Just thrilled for Alex and that obviously gave us an unbelievable start.

“Then Ajla has gotten better and better with each match she's played. For her to get the win tonight was very well deserved. She's been fantastic around the team all week. Then obviously the mixed doubles team, pretty special for them to go 3-0 here in Perth, their hometown.”