Defending champion Andy Murray added his voice to the growing list of players unhappy at the state of Wimbledon's grass courts, even claiming the hallowed Centre Court has developed divots.

World number one Murray said the courts were not in as good a condition as they have been in the past.

Other players have criticised the state of Court 18 with France's Kristina Mladenovic claiming that a hole had appeared in the surface.

Weeks of hot weather and little rain have left many surfaces stripped of grass after just five days of the two-week long tournament.

"The court I don't think is in as good of condition as previous years," said Murray after beating Italy's Fabio Fognini 6-2, 4-6, 6-1, 7-5 on Centre Court on Friday to reach the last-16 for a 10th straight year.

"There's quite a few spots on the court, like just behind the baseline and just in front of the baseline, where there's quite big lumps of grass, sort of almost like little divots there, which I don't remember really being the case."

Fognini agreed, saying the court "was really bad".

On Court 17 on Thursday, American star Bethanie Mattek-Sands suffered a horrific right knee injury after her leg buckled.

It was not clear, however, whether the state of the surface was a contributing factor in the 32-year-old's accident.

Earlier Friday, Wimbledon officials dismissed concerns over the state of the courts.

"The court preparation has been to exactly the same meticulous standard as in previous years," said the All England Club.