PARIS (AFP) - Defending women's singles champion Serena Williams was sensationally dumped out of the French Open on Wednesday in the biggest upset of the tournament so far.

The 17-time Grand Slam champion slumped to a shocking 2-6, 2-6 defeat by Spanish world No. 35 Garbine Muguruza in just over an hour.

The American had not been knocked out of a Grand Slam tournament before the fourth round since losing in the first round of the 2012 French Open.

The defeat comes less than an hour after her sister Venus was also knocked out, this time by teenager Anna Schmiedlova of Slovakia 6-2, 3-6, 4-6.