Borna Coric earned some badly-needed confidence with a big upset over world number four Dominic Thiem in stifling hot conditions at the ATP Cup Saturday as Austria slumped to defeat against a battling Croatia.
Thiem had won all three previous encounters with the 28th ranked Coric but failed to rediscover that form in Sydney, crashing 7-6 (7/4), 2-6, 6-3.
Veteran Marin Cilic, a finalist at the 2018 Australian Open, earlier used his experience to down Austria's Dennis Novak 6-7 (7/4), 6-4, 6-4, with Croatia taking an unassailable 2-0 lead in the Group E tie at the 24-team event.
"It was really hot in the beginning but in the second set I started to feel better because I think the temperature went down," said Coric.
"But it was really physical. It's really nice to beat him because at the end of last season I was not playing well and this will give me a lot of confidence.
Coric had six losses in a row at the end of 2019 but dug deep against Thiem, who worked six break points in the third set but couldn't convert any of them.
Former world number three Cilic showed similar fight in his tough three set victory.
"I'm very excited. It was absolutely very difficult," said Cilic, who has fallen to 39 in the world, his lowest since 2013.
"Dennis played a great level. It was our first meeting, and it was absolutely a thriller and difficult match... and also a great match to start the year. I think the level was really, really good."
Cilic sent down 15 aces and won all of his service games after losing the opening set.
Earlier in Group E, Poland's top player Hubert Hurkacz stunned Argentinian pocket-rocket Diego Schwartzman, who blamed searing heat for his defeat.
Temperatures soared well above 40 Celsius (104 Fahrenheit) in Sydney, although fears that smoke haze from bushfires burning across New South Wales could impact the action proved unfounded.
World number 14 Schwartzman, one of the smallest players on tour at just 5ft 7ins (1.70 metres), looked destined for victory after winning the first set but wilted 4-6, 6-2, 6-3.
Guido Pella, ranked 25, had earlier battled past Kamil Majchrzak 6-2, 2-6, 6-2 to give Argentina the edge in their tie.
Hurkacz returned to partner two-time Grand Slam winner Lukasz Kubot in a deciding doubles rubber, but his energy was sapped and they fell 6-2, 6-4 to Maximo Gonzalez and Andres Molteni, handing Argentina a 2-1 victory.
"The first match with this weather, you know, 45 degrees inside the court, difficult to play, difficult to be ready every single point," said Schwartzman, who has improved his year-end ranking in four straight seasons.
"Then I think he deserved to win. He played much better than me.
"You know, that part of the game with this weather, it's difficult to do every point. So every single point was difficult for me at the end," he added.