World number one Angelique Kerber's preparations for the defence of her Australian Open title were dealt a blow as she was knocked out in the second round of the Apia International in Sydney.

Kerber was well below the form that saw her triumph in Melbourne 12 months ago and then at the US Open later in 2016, losing serve five times as she suffered a shock 7-6 (7/5) 6-2 defeat to Russia's Daria Kasatkina.

With the year's first major less than a week away, Kerber will be under pressure having won only once in three matches this year, with her latest defeat following on from a surprise quarter-final exit in the Brisbane International last week.

The 28-year-old German's setback was the latest in a string of upsets as three other seeds crashed out in Sydney.

Slovakia's Dominika Cibulkova, seeded third, went down 6-4 6-3 to Canadian wildcard Eugenie Bouchard, while Anastasia Pavlyuchenkova saw off fellow Russian and fifth seed Svetlana Kuznetsova 7-5 6-3.

Ninth seed Roberta Vinci was brushed aside by Barbora Strycova, who claimed a 6-2 6-3 victory, but there was better luck for Caroline Wozniacki, the 10th seed defeating Yulia Putintseva 6-0 7-5.

Kasatkina will now face Britain's Johanna Konta in the third round (Getty)

Britain's Johanna Konta is the only seed left in the top half of the draw as she battled to a 6-1 6-3 victory over Russian-born Australian Daria Gavrilova.

The 25-year-old did not look back after breaking in the opening game and dropped only four points on serve in a first set she dominated.

Konta quickly established herself in the second set and while the plucky Gavrilova staved off four match points on serve in the eighth game, the British number one saved a break point and then sealed the win at the sixth attempt.

The sixth seed now faces Kerber's conqueror Kasatkina - the Russian ranked 16 places below Konta at 26 in the world.

Second seed Agnieszka Radwanska struggled in the first set of her clash with Christina McHale but found her rhythm to win 7-6 (7/1) 6-1, while China's Duan Yingying also progressed with a 6-3 6-2 win over American Coco Vandeweghe.

Konta beat Russia's Daria Gavrilova in straight sets (Getty)

Third seed Monica Niculescu made it through to the third round of the Hobart International with a 6-3 6-2 success against Belgian Kirsten Flipkens, while Veronica Cepede, Elise Mertens and Jana Fett also progressed.