Venus Williams could not maintain her excellent Australian Open form, falling to a straight-sets defeat at the hands of Kristina Mladenovic in the St Petersburg Ladies Trophy.

The veteran rolled back the years at Melbourne Park last week, embarking on a run to the final where she was beaten by younger sister Serena.

And that campaign possibly took its toll on the 36-year-old, who enjoyed a bye in the first round in Russia but was well below her best on Thursday.

She was broken five times as Mladenovic wrapped up a 6-3 6-1 success in less than an hour, victory secured when Williams double-faulted to sum up a disappointing outing in fitting fashion.

Mladenovic will face defending champion Roberta Vinci in the quarter-finals, after the Italian eased past Andrea Petkovic 6-4 6-4.

Meanwhile, Fifth seed Elena Vesnina had to come from a set down to defeat Alize Cornet 2-6 6-3 6-3.