Samantha Stosur has suffered a shock straight-sets loss in the opening match of her US hardcourt campaign at the Stanford Classic in California.

The second seed went down 6-2 6-4 to world No.83 Olga Govortsova of Belarus in the round-two clash on Tuesday, after having enjoyed a bye in the first round.

The defeat is a huge blow for the Australian, with barely a month left before the US Open – the title she won back in 2011.