Who will be more excited when they face off against each other - Roger Federer or Serena Williams?
The tennis superstars will play each other for the first time when Switzerland take on the US in the Hopman Cup mixed doubles on Tuesday.
The match-up is set to draw worldwide interest, and the players themselves are just as excited.
Williams has already declared her intent to get a selfie with Federer to post on Instagram.
"It's the age of social media," Federer said with a laugh when told about Williams' plan.
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Federer lavished praise on the 23-time grand slam winner, saying the upcoming match will be something special.
"It's very exciting for both of us," Federer said.
"I hope a lot of tennis fans will tune in to watch the match, because it's going to be one time and probably never again in this kind of a competition.
"I admire everything she's done on and off the court.
"We can relate to one another now even more so for being parents.
"I've played with some of the greatest doubles players in the world in Martina Navratilova and Martina Hingis, and I've also played against some of the best in Venus and other great ones in the past.
"This time it's against Serena, who is one of the biggest champions ever in our sport - men and women combined."
Federer opened his Hopman Cup campaign in perfect style, crushing world No.91 Cameron Norrie 6-1 6-1 in 57 minutes.
The 37-year-old will face off against rising star Frances Tiafoe in Tuesday's tie against the US, before playing Williams in the mixed doubles.
Williams will take on Belinda Bencic in the women's singles rubber.
Bencic beat Katie Boulter 6-2 7-6 in her opening clash of the tournament to help Switzerland clinch victory over Great Britain.
