Australians Samantha Stosur and Daria Gavrilova have been knocked out of the Charleston Open in South Carolina.

Sixth seed Stosur hit 24 unforced errors in her 7-5 6-3 loss to Romanian world No.33 Irina-Camelia Begu in the round-of-16 clash on Thursday (Friday AEST).

Begu sent down five aces to Stosur’s one in the opening set before breaking the 2011 US Open winner twice in the second to coast to a straight-set victory.

World No.27 Gavrilova went down to in Russian Daria Kasatkina 6-3 4-6 6-0.

Both players struggled to find a rhythm on serve in a see-sawing opening two sets.

The unseeded Kasatkina saved all four of Gavrilova’s break points in the decider as the Australian continued her serving woes.

Australian qualifier Anastasia Rodionova meets fifth seed Caroline Wozniacki on Thursday.

In other results, Latvia’s eighth Anastasija Sevastova did it the hard way against Tunisia’s Ons Jabeur 7-5 7-6 (8-6) and Mirjana Lucic-Baroni of Croatia beat Dutch seventh seed Kiki Bertens 7-6 (7-5) 6-4

Fanny Stollar was forced to retire from her match against Jelena Ostapenko.