Alex de Minaur came from a set and a break down to overhaul Germany's Alexander Zverev and cause a major upset at the inaugural ATP Cup in Brisbane on Friday.
World number seven Zverev seemed to be cruising to victory when he won the first set and broke the Australian in the first game of the second.
However, the towering German's serve went off the boil and de Minaur stormed back to win 4-6, 7-6 (7/3), 6-2 -- helped in no small part by Zverev's 14 double faults -- and give Australia an unassailable 2-0 lead in their Group F tie.
Earlier, the mercurial Nick Kyrgios laid on a serving master class to beat Jan-Lennard Struff.
Kyrgios, who has promised to donate Aus$200 (US$140) for every ace he serves in the tournament to people affected by the Australian bushfires, fired 20 aces past Struff to win 6-4, 7-6 (7/4).
He became emotional on court when talking about the bushfires.
"We've got the ability and the platform to do something," he said. "My home town is Canberra and we've got the most toxic air in the world at the moment. That's pretty sad. It's tough."
In the opening Group F tie, Canadian young gun Denis Shapovalov edged world number six Stefanos Tsitsipas in his country's clean sweep against Greece.
The 20-year-old Shapovalov downed the world number six 7-6 (8-6), 7-6 (7/4) after Felix Auger-Aliassime thrashed Michail Pervolarakis 6-1, 6-3.
The two then combined in the doubles to beat Pervolarakis and Tsitsipas 6-2, 6-3.
"It was definitely a great start to the event -- these boys played awesome," Canadian captain Adriano Fuorivia said. "It's easy to support them when they're playing like that."
Shapovalov's left-handed serve was working beautifully in his match against Tsitsipas, who faltered at key moments in both tiebreaks.
Tsitsipas, 21, had a superb end to 2019, winning the ATP Finals in his tournament debut and reaching a career-high world ranking of five.
He went into the match as slight favourite over the Canadian, ranked nine places below him.
Both players were relatively untroubled on serve but Shapovalov made the clutch points in the two tie-breakers to take the match in just over two hours.
"It's definitely a huge win for me," Shapovalov said.
"Obviously, he had an unbelievable end to the season and he's definitely one of the top players in the world right now so to beat a guy like this first match of the year, it's really special for me."
Tsitsipas conceded he had returned poorly, which proved the difference.
"I don't know whether it was because he was a lefty. I don't really usually have this much trouble playing lefties, but today I think he was serving well -- that was pretty much it," Tsitsipas said.
World number 21 Auger-Aliassime was far too strong for Pervolarakis, 466 places further down the ATP rankings.
The Canadian cruised through the first set before Pervolarakis -- playing his first match on the main ATP tour -- could adjust to the pace, but the 19-year-old was never in trouble and wrapped up the match in 69 minutes.