I felt nothing on the face of the match and then it just became worse every second and I felt extremely dizzy and extremely tired, " says Serena Williams in a statement via the tournament organizer.
For spanjorskan Muguruza now waiting French huguenots, frisians Kiki Bertens, who beat brittiskan Johanna Konta in straight sets in the fourth round.
Even the world number Simona Halep is on. The Romanian language had it tougher than expected against kvalspelaren Kateryna Kozlova from Ukraine, but eventually won with 2-0 in sets, 7-6, 7-5.