Ivanovic's reign as world number one did not last very long this year

Former world number one Ana Ivanovic's poor run of form continued when she crashed out of the Pan Pacific Open at the second round stage on Thursday.

The 20-year-old has struggled since winning the French Open in May and lost in three sets to Nadia Petrova.

Ivanovic, who has suffered from a thumb injury in recent months, slipped to a 6-1 1-6 6-2 defeat to the Russian.

In the quarter-finals on Friday, Petrova will play Agnieszka Radwanska of Poland, who beat Marion Bartoli.