Rusty world No.1 Angelique Kerber has stumbled in the lead-up to her Australian Open defence with a quarter-final loss to Elina Svitolina at the Brisbane International.

Ukrainian sixth seed Svitolina, who beat Serena Williams at the Rio Olympics, upset the Australian and US Open champion 6-4 3-6 6-3 at Pat Rafter Arena.

In a thrilling final set, the world No.14 came from a break down to clinch the match in almost two hours and set up a semi-final clash with Karolina Pliskova.