Poland’s top player Hubert Hurkacz stunned pocket-rocket Diego Schwartzman at the ATP Cup Saturday, with the Argentinian blaming 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 to a 4-6, 6-2, 6-3 loss.

Guido Pella, ranked 25, had earlier battled past Kamil Majchrzak 6-2, 2-6, 6-2 to give Argentina the edge in their Group E tie at the 24-nation event.

Hurkacz returned to partner two-time Grand Slam winner Lubasz Kubot in a deciding doubles rubber, but his energy was sapped and they slumped 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.

Hurkacz, who won his first title last year at Winston-Salem and earned the biggest win of career over world number four Dominic Thiem in Miami, again proved he has the ability to beat high-ranked players.

Almost a foot taller than the Argentinian, the world number 37's big serve was a key weapon. Pella beat Majchrzak in front of a noisy Argentinian contingent at Ken Rosewall Arena. With ice towels out at the breaks to cool him down, he broke serve five times in his two-hour, four-minute triumph.

“I’m pretty happy overall. First of all, because I think I played a very good match in the first and third set," he said after the baseline battle.