Throwback to when Serena Williams called Roger Federer the GOAT at Rally for Relief charity event

Serena Williams is arguably the greatest tennis player in the world, and one of the greatest athletes of modern times. But during a charity event in 2020, Serena Williams delighted Roger Federer fans across the world when she referred to the Swiss as the GOAT.

The 2020 Australian Open was marred by the bushfire catastrophe. Unusually large bushfires raged across Australia from June 2019 until March next year, severely impacting the lives of its citizens.

Despite that, the Australian Open went ahead as scheduled from 20 January to 2 February. Five days prior to the start of the tournament, some of the biggest names in the world of tennis came together for the "Rally for Relief" charity event staged for Australia's benefit.

Building on Nick Kyrgios' idea of donating $200 for every ace hit during the summer of tennis that year, the exhibition event took place at Rod Laver Arena to a packed crowd. All the proceeds from the night, almost $5 million, were donated to the Victorian Bush Appeal for rehabilitation and relief.

Caroline Wozniacki was one of the team's captains while Serena Williams served as the other. Coco Gauff, Alexander Zverev, Stefanos Tsitsipas and Naomi Osaka were part of Team Wozniacki. Team Williams, on the other hand, comprised of Rafael Nadal, Dominic Thiem, Petra Kvitova and Novak Djokovic.

After the doubles matches, the teams called on the singles players they had selected to face off against each other. Wozniacki announced that Kyrgios would be their chosen champion. The moderator then asked Serena Williams to give a hint regarding who theirs was.

The 23-time Grand Slam champion commented that they would need the services of someone as good as the GOAT to keep up with Kyrgios, referring Federer who walked out to a thumping reception seconds later.

"I wasn't expecting [Wozniacki] to bring out Nick [Kyrgios]. That is totally not fair," Serena said. "I don't know how I can beat that, but you know, how about the GOAT?"

Roger Federer went on to win the match in a tiebreaker to seal the tie in favor of Team Williams.

Serena Williams and Roger Federer had mixed fortunes at the 2020 Australian Open

Serena Williams suffered a shock defeat in the third round of the 2020 Australian Open

At the 2020 Australian Open following the "Rally for Relief" event, Roger Federer and Serena Williams experienced contrasting fortunes. Serena Williams is gunning to become the first woman to win a Grand Slam in four decades and equal Margaret Court for most Grand Slam titles. But her campaign ended in a three-set loss against Qang Wiang in the third round.

The exit marked the former World No. 1's worst result in the tournament since 2006, barring her absence in 2018. The 40-year old had won the title on four occasions since then, most recently in 2017.

Meanwhile, Roger Federer progressed as far as the semifinals, where he lost to eventual winner Novak Djokovic in straight sets. His third-round victory against John Millman marked his 100th win at the Melbourne major. To this day, the Swiss is the only person to have won 100 matches at the Australian Open.