Tennis royalty Roger Federer and Serena Williams have been pounding balls across the net for more than 20 years and they are back for yet another season, starting in Perth at the mixed teams Hopman Cup from Saturday.

Both 37 years old and with 43 Grand Slam singles titles between them, Federer is now a father and Williams a mother but tennis keeps calling, with neither showing any sign of quitting.

Flying Federer leads Switzerland in defence of the Hopman Cup title he won alongside Belinda Bencic earlier this year, with the pair teaming up once again.

Williams returns to the country for the first time since her famous Australian Open win in 2017, when eight-weeks pregnant with baby Alexis Olympia.

She partners rising American star Frances Tiafoe in her fifth visit to Western Australia.

In a mouthwatering prospect, Switzerland play the United States on January 1 with two of the greatest players ever facing each other in the mixed doubles.

Federer conceded it was a once-in-a-lifetime match-up and one he was looking forward to.

“We’ll probably play it down a little bit and say it’s not that big of a deal for us, it’s just another tennis match, but it really isn’t because it’s probably going to happen once and never again,” he said on Thursday.

The Swiss great has spent the off-season training hard ahead of the defence of his Australian Open title and said he was happy with his fitness.

“The last three or four weeks have been very intense. I’m very excited and motivated for this next season.”

The last two seasons the Swiss legend started in Perth, he went on to win the opening Grand Slam of the year.

Fit and primed

Williams played an exhibition match against sister Venus in Abu Dhabi this week, but Perth will be her first competitive event since her meltdown in the US Open final this year.

She received a code violation for coaching, a penalty point for racquet abuse and a game penalty for calling the umpire a “liar” and a “thief” during her defeat by Naomi Osaka.

Federer later said Williams “went too far” and should have walked away, adding an edge to their upcoming encounter.

Williams opted not to talk about the rant in Abu Dhabi, instead preferring to look forward.

“My fitness, I feel like I’m pretty fit,” she said, adding that she was excited by another season as she primes to win an eighth crown at Melbourne Park to match the record 24 Grand Slam titles held by Margaret Court.

“I haven’t played with Frances in mixed doubles before, but have watched him from afar this year. He has a very bright future and I’m looking forward to being on court with him, it will be a lot of fun,” she said.

Tiafoe, 20, is considered one of the world’s most impressive Next Gen stars.

He won his first singles title in February to become the youngest American champion on tour since Andy Roddick in 2002 and hit a career high ranking of 38 in August.

“I’m extremely nervous to play with Serena, a complete legend,” he admitted. “But I’m sure it’ll be fine.”

Germans Alexander Zverev and Angelique Kerber, the beaten 2018 finalists, form a formidable partnership in the way of Switzerland and the United States.

Zverev had a breakout 2018 season and is now ranked world number four, one behind Federer, while Kerber is world number two after winning Wimbledon, where she beat Williams in the final.

Two-time Grand Slam champion Garbine Muguruza partners David Ferrer for Spain while perennial favourites France are back with Lucas Pouille and Alize Cornet.

Australia (Matt Ebdon/Ashleigh Barty), Greece (Stefanos Tsitsipas/Maria Sakkari) and Britain (Cameron Norrie/Katie Boulter) make up the field.