American qualifier Bethanie Mattek-Sands dumped top seed and defending champion Agnieszka Radwanska out of the Sydney International on Tuesday.
Mattek-Sands, ranked 48, upset the world number five 7-5, 6-2 in just over 90 minutes to disrupt Radwanska's lead-up to next week's Australian Open in Melbourne.
Radwanska's defeat was the first time since 1998 that the women's titleholder has lost her opening match in Sydney.
"She was really playing aggressive today. It didn't matter if it was my first serve, second serve, or anything, she was really going 100 per cent for every ball," Radwanska said.
Former Wimbledon champion Petra Kvitova is the highest seed left in the draw after ousting American Christina McHale for the loss of just one game.