Sixth seed Samantha Stosur was unable to maintain the Australian charge at the Charleston Open in South Carolina, losing in straight sets to Romania’s Irina Begu.

Stosur joined compatriots Anastasia Rodionova and Daria Gavrilova in the round of 16 with a straight sets win over 2008 US Open winner Jelena Jankovic but was unable to repeat her form on Thursday, falling 7-5 6-3 to the 10th seed.

Begu hit five aces to one in the opening set before proving too strong for the 2011 Flushing Meadows winner.

In other results on Thursday former world No.1 Caroline Wozniacki continued her promising start to the season with a straight sets win over Germany’s Annika Beck, while Latvia’s eighth Anastasija Sevastova did it the hard way against Tunisia’s Ons Jabeur 7-5 7-6 (6).

Elsewhere Fanny Stollar was forced to retire from her match against Jelena Ostapenko, while Croatian 11th seed Mirjana Lucic-Baroni was too good for seventh seed Kiki Bertens of the Netherlands.