Top-ranked Angelique Kerber stunned by Russian teen Daria Kasatkina in Sydney

Russian teenager Daria Kasatkina put Angelique Kerber’s Australian Open preparations in a spin with a straight-sets upset win over the world number one at the Sydney International on Tuesday.

The 19-year-old, who is ranked 26 in the world, underlined her huge potential with a confident performance to beat 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 last week.

Kerber, who had a breakthrough 2016 after winning two grand slams on her way to becoming world number one, struggled to find any rhythm and her usually strong forehand was astray.

Asked about her emotions after her biggest win yet, Kasatkina said: “Difficult to explain because I beat the number one 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 sizzledwith defending champion Svetlana Kuznetsova and last year’s finalist Dominika Cibulkova both knocked out in the second round.

World number nine 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.

Former Wimbledon finalist Bouchard, who has now won consecutive matches at a tournament for the first time since August, stretched her head-to-head record over Cibulkova to 4-1 and will face Pavlyuchenkova in the last eight.

“Any time you play one of the best players in the world it’s like a standard of where you’re at,” Bouchard said.

Dominika Cibulkova and Caroline Wozniacki win in Sydney International openers

“I have taken a couple of solid steps this week, but I’m far from where I want to be. Even though I won, to me, I could do a lot of things better.”

World number 27 Pavlyuchenkova, who knocked out Australian number one Samantha Stosur in the first round, won in her first career meeting with Kuznetsova.

Ninth-seeded Italian Roberta Vinci was another seed to fall after losing 6-2, 6-3 to veteran Barbora Strycova of the Czech Republic.