'Oh what a night'! What Serena, Federer said after dream clash

It took 43 Grand Slam triumphs and over two decades to set up the on-court meeting but when the moment finally arrived, Roger Federer and Serena Williams gave the capacity crowd at Perth a night to remember.

Federer's Switzerland beat Serena's US 4-2, 4-3 (3) in the mixed doubles decider at the Hopman Cup on Tuesday.

The duo hailed each other in the post-match interview before the tennis titans captured the iconic moment in what Hopman Cup on Twitter described as the 'greatest selfie of all time'.

Federer also posted the picture on social media, with the caption, "Oh what a night."

"He's the greatest of all time, to be honest." 23-times Grand Slam winner Williams said.

"It was so fun to do this at the pinnacle of our careers.

"Both on and off the court, he has such charisma, he's such a wonderful player. Maybe I could get some tips from him later on."

Federer and Williams have won a combined 171 career titles and have spent over 600 weeks at world number one.

"It was great fun," Federer added of the match.

"You see how determined and focused she is, and I love that about her."

Defending champions Switzerland take on Greece in their final round-robin match on Thursday while the US face Great Britain's Cameron Norrie and Katie Boulter.