Angelique Kerber was bundled out of the Hong Kong Open as Daria Gavrilova beat the world number one to move into the semi-finals on Friday.
Kerber has work to do to ensure she finishes the year at the top of the rankings after slumping to a 6-3 6-1 loss against Australian, ranked 38th in the world.
The US Open and Australian Open champion failed to match the high standards she has set in the best year of her career, making a string of unforced errors in a bad day at the office.
Top seed Kerber, beaten by Elina Svitolina in the third round of the China Open last week, struggled on serve, double-faulting six times and winning only 43 per cent of points on her first serve and failing to hold on six occasions.
Eighth seed Gavrilova punished the German, beating her for the first time in just an hour and 14 minutes to seal a semi-final meeting with Kristina Mladenovic, who overcame Bethanie Mattek-Sands 3-6 6-3 7-6 (7-3)
Kerber said: "It's a long season and we've been playing for 10 months. I've been playing a lot of matches this year, and I think I need a few days off to get ready for Singapore [the WTA Finals] because I am tired and I need a lot of treatment to get ready for the next one.
"I was not serving at my best, I made a lot of double faults which is not normal for my game."
Caroline Wozniacki remains in the hunt for a second title in as many months after beating Wang Qiang 6-3 7-5, with either Jelena Jankovic or Alize Cornet set to face the former world number one at the semi-final stage.