Swiss Timea Bacsinszky has fought back to deny Samantha Stosur a rare foray into the Sydney International quarter-finals.

Bacsinszky rallied from a set down to defeat Stosur 5-7 6-4 6-3 in a seesawing encounter stretching a minute shy of two and a half hours.

Despite Stosur missing out on only a third last-eight place in 12 visits to Sydney, the epic work-out was a perfect warm-up for the Australian Open starting in Melbourne on Monday.

Bacsinszky is certain to meet a qualifier next, either Australian youngster Priscilla Hon or Belarusian Aliaksandra Sasnovich.

Belgian 10th seed Elise Mertens also needed three sets on Wednesday to beat Czech qualifier Katerina Siniakova 7-6 (7-4) 2-6 6-2 in a first-round match.