Melbourne - Roger Federer might be one of the greatest players ever, but even he was bowled over by Serena Williams when they played mixed-doubles at the recent Hopman Cup.

The Swiss star admitted he was flustered when serving to the American 23-time Grand Slam winner, saying she had an "aura".

"It was a wonderful atmosphere (playing against Serena), one I'd been looking forward for some time," he said after moving into the Australian Open third round.

"I had a bad serve percentage against her ... it must have had something to do with Serena's aura. I'm not kidding. She's a wonderful player. But we won in the end so I was happy about that, too!"

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Meanwhile, young American Frances Tiafoe stated the obvious when asked what it was like when he had the opportunity to play with Williams at the Hopman Cup.

"I mean, off the court, she's so chilled and relaxed. Once she gets on, she hates losing. It's unacceptable," he said of the 23-time Grand Slam champion after making the Australian Open third round.

Tiafoe is only 20-years-old and said it was nerve-wracking when he first teamed up with perhaps the greatest women's player ever.

"I was about as nervous as could be. But I think we grew a pretty good relationship."