Jason Kubler has opened up about a heartwarming text exchange with Nick Kyrgios following his stirring upset win in the United Cup as he spurred the Australian team to an upset win over Spain.

Ranked outside the top 100, Kubler was called in to replace Kyrgios at the last minute and has miraculously won both of his singles matches - against Britain's Dan Evans, then Spain's Albert Ramos-Vinolas.

Kubler revealed details of his text exchange with Kyrgios after the 29-year-old Queenslander's stirring 6-3 4-6 6-3 victory on Tuesday night.

'I actually messaged him after my match against Evo (Evans),' Kubler said.

'I messaged him, 'Thank you so much for the opportunity'. Basically (said), 'I hope your body gets better, and we can have a hit before the Australian Open.

'That's the way I see it. He could have played, and he could have done unbelievably well, like I know we all think he can.

'Fortunately, he gave me the opportunity to play, and I have sort of taken it with both hands and run with it.'

Elsewhere, Samantha Stosur held her nerve amid fierce thunder and lightning to help seal Australia's historic first United Cup victory.

Stosur teamed with fellow veteran John Peers to defeat Jessica Bouzas Maneiro and David Vega Hernandez 6-2 6-3 in the deciding mixed doubles rubber to earn Australia a 3-2 comeback victory over Spain on Tuesday night.

At one point early in the second set it seemed the winner-takes-all rubber would be suspended after a series of lightning bolts in the surrounding suburbs threatened to blow the roof off Ken Rosewall Arena.

The players, including mother-of-two Stosur who had her children on site, shuddered in fear as the crackling of thunder exploded seemingly all around them.

Eager to spend as little time on court as possible, Stosur - Australia's playing co-captain - and Peers wrapped up the victory inside an hour.

'It was a little bit nerve-wracking, I won't lie,' Stosur said.

Earlier, Kubler's heroics against Ramos-Vinolas sent the tie into a deciding mixed rubber.

Kubler's win over the higher-ranked Albert Ramos-Vinolas made it 2-2 after emerging star Olivia Gadecki lost 6-2 6-2 to the 193rd-ranked Bouzas Maneiro in an inauspicious Cup debut.

The dead-rubber tie was all square at one match apiece after Alex de Minaur upset Rafael Nadal on Monday night, before Maddison Inglis lost to Nuria Parrizas Diaz.

A late call-up to replace the injured Zoe Hives, Gadecki was making her first appearance in the green and gold since a Billie Jean King Cup doubles debut in November 2021.

The former teenage prodigy was headed for stardom before controversially sitting out the last Australian summer - including the 2022 Australian Open - after declining to be vaccinated against COVID-19.

Formerly coached by David Taylor, the esteemed ex-mentor of Samantha Stosur, Alicia Molik, Martina Hingis and Naomi Osaka, Gadecki made a swift rise up the rankings to a career-high No.156 in July last year.

Ash Barty has also taken Gadecki under her wing and will be courtside for her AO main-draw debut at Melbourne Park, after the 20-year-old was handed a wildcard entry from Tennis Australia.

While Gadecki has fallen out of the women's top 200, Kubler is set to soar to a career-high ranking of around 80th next Monday following his two wins in Sydney.

'The feelings I have had on court are crazy,' Kubler said.

'I'm from Brisbane so I thought Sydney wouldn't be the place (I'd get support). It's unbelievable. I couldn't imagine anything like this.'

The 29-year-old's new standing is nothing less than he deserves, after he was Australia's last player standing at the French Open and then reached the round of 16 at Wimbledon in July.

But he was cruelly denied a swag of rankings points after the ATP opted against awarding them at Wimbledon in protest at the All England Club's ban on Russian and Belarusian players because of Vladimir Putin's war on Ukraine.

'I'm honestly just grateful to represent Australia again,' Kubler said.

'Last year I was lucky enough to play Davis Cup. I just jumped at the opportunity to play for Australia with these guys.

'We are such close friends and I love going to war with them.'