Madrid - Caroline Wozniacki's hopes of returning to the world number one spot were dashed on Wednesday by a surprise defeat to Dutchwoman Kiki Bertens at the Madrid Open.
Wozniacki would have taken top spot from Simona Halep had she won the title this week, but the Dane saw Bertens progress to the quarter-finals instead thanks to a 6-2, 6-2 victory.
After winning her first Grand Slam at the Australian Open in January, Wozniacki moved back to the summit of the rankings for the first time since 2012.
But clay is not the 27-year-old's favourite surface and she struggled to cope with the craft of Bertens, whose five WTA titles, the latest in Charleston last month, have all come on the red dirt.
Bertens will now face either Maria Sharapova or Kristina Mladenovic for a place in the semi-finals.
The world number 16 reached the quarter-finals at the French Open in 2016 before losing to Serena Williams.