The inaugural United Cup will kick off the 2023 tennis season on Thursday and has already attracted some of the best players in world tennis with the eye-watering payouts it is offering.

The mixed gender $15m tournament, which will compete across Sydney, Perth and Brisbane over 11 days, has attracted 20 of the best male and female players from around the globe.

Players competing include Rafael Nadal, Alex Zverev, Stefanos Tsitsipas from the ATP tour, and Iga Swiatek, Madison Keys, Maria Sakkari and Caroline Garcia from the WTA.

Tennis officials will be offering $3.5million in appearance fees alone, with the top players looking to make more at the United Cup for showing up than they would reaching the second round at the Australian Open. 

The players ranked in the top ten will pocket $US200,000 for just playing their group matches - and players ranked 11-20 will walk away with $US100,000.

Every single match played during the tournament has prize money attached, which is why players from around the world are flocking to the event.

Top Italian player Matteo Berrettini acknowledged the attraction of the big-money tournament when questioned about it in front of teammates this week.

'Everybody smiled when you said that, see, so it's good,' Berrettini told Newscorp.

'Obviously we don't just play for the money but it's important that it's there.'

The 26-year-old explained that he is also excited by the nature of the tournament, too.

'I've never even played mixed doubles so it's going to be exciting,' he said. 

'I'm curious about it and it's a really cool format.'

The United Cup is the first joint ATP-WTA mixed tournament - where some of the best men and women in the sport will play alongside each other, in teams and against each other in the mixed doubles.

18 nations will be split into six pools of three teams, with finals played in Sydney to finish the event. 

The United Cup replaces the ATP Cup - which was a men's only tournament that struggled to attract top talent after the COVID-19 shut down.