Sydney (AFP) - Russian teenager Daria Kasatkina put Angelique Kerber's Australian Open preparations in a spin with a straight-sets upset win over the world No. 1 at the Sydney International on Tuesday.

The 19-year-old, ranked No. 26, underlined her huge potential with a confident performance to take out the German, 7-6 (7-5), 6-2 in a second-round match played in sweltering conditions.

Only last week Kasatkina held a match point before going down to French Open champion Garbine Muguruza in the second round at the Brisbane International.

While she continues her march, Kerber's form is a concern ahead of next week's opening Grand Slam of the year, having also lost to Ukraine's Elina Svitolina 6-4, 3-6, 6-3 in the quarter-finals of the Brisbane International.

Kerber, who had a breakthrough 2016, winning two Grand Slams on her way to becoming world No. 1, struggled to find any rhythm and her usually strong forehand was astray.

Asked about her emotions after her biggest win yet, Kasatkina said courtside: "Difficult to explain because I beat the No. 1 in the world and it doesn't happen every day. I think I got some confidence."

It was a day of upsets in Sydney where temperatures sizzled around 36 deg C, with defending champion Svetlana Kuznetsova and last year's finalist Dominika Cibulkova both knocked out in the second round.

World No. 9 Kuznetsova went out to Russian compatriot Anastasia Pavlyuchenkova 7-5, 6-3 while Canada's Eugenie Bouchard continued her return to form with a 6-4, 6-3 win over the sixth-ranked Cibulkova.