Daria Kasatkina claimed the scalp of out-of-sorts world number one Angelique Kerber and Dominika Cibulkova was among the other seeds to be sent packing in the second round of the Sydney International on Tuesday.

Kerber has failed to hit the ground running this season after a stellar 2016 and the German was beaten 7-6 (7-5) 6-2 by unseeded Russian Kasatkina in her final tournament before starting the defence of her Australian Open title next week.

Kasatkina, 19, gave another example of her potential by breaking the top seed five times in a match she finished with an impressive first-serve percentage of 70 to seal her first triumph over a top-five opponent.

Kerber made far too many unforced errors and struggled to get her venomous forehand going, bowing out a week after a loss to Elina Svitolina in her second match at the Brisbane International.

Third seed Cibulkova also failed to build momentum before the first major of the year in Melbourne, going down 6-4 6-3 to wildcard Eugenie Bouchard.

Bouchard will play Anastasia Pavlyuchenkova in her first WTA quarter-final for 10 months after the WTA Finals champion was unable to live with the Canadian's positive approach.

Second seed Agnieszka Radwanska was a 7-6 (7-1) 6-1 winner against Christina Mchale after Pavlyuchenkova came through an all-Russian encounter with fifth seed Svetlana Kuznetsova. Ninth seed Roberta Vinci was consigned to an early exit by Barbora Strycova.

Johanna Konta will face Kasatkina after easing past Daria Gavrilova 6-1 6-3, while qualifier Duan Ying-Ying upset Coco Vandeweghe in the other second-round match.

Third seed Monica Niculescu is through to the last eight at the Hobart International as a result of a 6-3 6-2 success against Kirsten Flipkens.

Lucky loser Veronica Cepede Royg required five match points to dump out Andrea Petkovic, while there were also wins for Jana Fett and Elise Mertens.