Two-time Australian Open champion Victoria Azarenka has been knocked out by world number 61 Jessica Pegula in one of the biggest upsets of the tournament so far.
Azarenka, the 12th seed, was well short of her best in the 7-5, 6-4 loss, and called for a medical timeout during the second set when she appeared to be short of breath.
She continued after receiving treatment by way of an asthma inhaler, but went on to make 25 unforced errors and served seven double faults in a flat performance.
Meanwhile, a year after reaching the final of the Australian Open, Garbiñe Muguruza started in impressive fashion at Melbourne Park.
She dismantled Russia’s Margarita Gasparyan without really breaking a sweat, winning 6-4, 6-0.
The 14th seed made just two unforced errors as she tore through the second set in 27 minutes to continue her impressive run on Australian soil since the start of last year.
The Spaniard has won 13 matches down under in that time, losing just twice, including the Yarra Valley Classic decider against world number one Ash Barty on the weekend and the final of last year’s Open, where she lost to fellow first-round winner Sofia Kenin.
Elsewhere on day two, fifth seed Elina Svitolina, the 11th-seeded Belinda Bencic and rising US star Cori Gauff all earned berths in the second round.
American Ann Li upset 31st seed Shuai Zhang 6-2, 6-0 thanks to 22 winners compared to Zhang’s 22 unforced errors.
On the men’s side of the draw, fourth seed Daniil Medvedev got past Vasek Pospisil 6-2, 6-2, 6-4.
Medvedev’s fellow Russian and seventh seed, Andrey Rublev, also won in straight sets, beating Germany’s Yannick Hanfmann 6-3, 6-3, 6-4.
Highly touted Spanish 17-year-old Carlos Alcaraz took down a fellow qualifier, Dutchman Botic van de Zandschulp, in straight sets.
He is in the same quarter of the draw as 22-year-old sixth seed Stefanos Tsitsipas, with a mouth-watering third-round clash on the cards for two of the brightest young stars in tennis.