Victoria Azarenka was knocked out of the French Open fourth round and could now lose her world number one tennis ranking.
The Belarussian Azarenka was beaten 6-2 7-6 by Slovakian 15th seed Dominika Cibulkova and must wait to see if Russian Maria Sharapova will claim the top spot.
Sharapova, who meets unseeded Czech Klara Zakopalova in the last 16 Monday and has yet to drop a set, must get to the Roland Garros final for the first time to become number one.
Elsewhere Italy's Sara Errani saw off her second French Open champion in two rounds when she beat Russian Svetlana Kuznetsova 6-0 7-5 to reach the quarter-finals for the first time.
Kuznetsova's demise left Li Na as the only French Open champion still in the women's draw, after earlier-round defeats for Francesca Schiavone and Serena Williams.
Angelique Kerber of Germany reached the quarter-finals for the first time with a straight sets wins over Petra Martic of Croatia.