Sofia Kenin lost to Madison Keys in an all-American first-round encounter in Melbourne ©Getty Images

Former Australian Open champion Sofia Kenin was the biggest casualty on the opening day of the Australian Open as home favourite Ashleigh Barty and Spain's 20-time Grand Slam winner Rafael Nadal stormed through to the second round.

Kenin, a winner of the women’s singles in 2020 and the 11th seed, crashed out to fellow American Madison Keys in straight sets.

Keys, ranked world number 51, claimed her first title in over two years at the Adelaide International 250 at the weekend and continued her fine form as she secured a 7-6, 7-5 victory in Melbourne.

"At this point every first round is tough, but when you have to go up against a Grand Slam champion it's never easy," said Keys.

"I think knowing that she was going to compete so well, I just had a really good mentality and attitude.

"I had a couple of opportunities I didn’t really capitalise upon and I was able to reset and continue to play well."

There were no such problems for last year's winner Naomi Osaka of Japan who cruised to a 6-3, 6-3 triumph against Colombia's Camila Osorio.

Osaka was making her first appearance at a Grand Slam since the 2021 French Open when she withdrew prior to her second match after refusing to participate in press conferences.

Crowds on the show courts have been capped at 50 per cent due to COVID-19 restrictions but those inside the Rod Laver Arena were treated to a fine showing from Barty.

The world number one took just 54 minutes to thump Ukraine's Lesia Tsurenko 6-0, 6-1, making a statement of intent to the rest of the field.

French Open champion Barbora Krejčíková of the Czech Republic also took little time to dispatch Germany's Andrea Petkovic, triumphing 6-2, 6-0.

There were also straight-sets wins for Olympic champion Belinda Bencic of Switzerland, Greece's fifth seed Maria Sakkari, two-time major winner Victoria Azarenka of Belarus and Ukraine's Tokyo 2020 bronze medallist Elina Svitolina.

Highly-rated American teenager Coco Gauff had a disappointing day at the office as she went down 6-4, 6-2 to China's Wang Qiang.

Cameron Norrie had a stunning 2021 which saw him rise to world number 12 and win the Indian Wells Masters.

However, the British number one has yet to hit top form in 2022 and was knocked out of the first round in Australia following a 6-1, 6-4, 6-2 defeat to Sebastian Korda of the United States.

With Serbia's nine-time champion Novak Djokovic out of the competition after his visa was cancelled, Nadal will be aiming to secure a record 21st men's singles Grand Slam title.

Playing in a major for the first time without Djokovic and Swiss star Roger Federer, Nadal was in cruise control as he hammered American Marcos Giron 6-1, 6-4, 6-2.

Germany's Alexander Zverev will also be looking to make the most of Djokovic's absence and the Olympic champion swept aside compatriot Daniel Altmaier 7-6, 6-1, 7-6.

Fellow top-10 seeds Matteo Berrettini of Italy and Poland’s Hubert Hurkacz both came through four-set tussles, beating American Brandon Nakashima 4-6, 6-2, 7-6, 6-3 and Belarus' Egor Gerasimov 6-2, 7-6, 6-7, 6-3, respectively.

Lucas Pouille, an Australian Open semi-finalist in 2019, fell at the first hurdle on this occasion as he lost 3-6, 6-3, 6-4, 6-3 to Corentin Moutet in an all-French opening-round clash.

There were also first-round exits for Italy’s Fabio Fognini and South Africa’s Kevin Anderson, losing in straight sets to Dutchman Tallon Griekspoor and American Reilly Opelka.

Competition is due to continue tomorrow with more opening-round matches.