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 No.1 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 an 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.

"Grass is a natural surface and it is usual for the baselines to start to be showing signs of wear and tear four days into the championships."

However, French 12th seed Mladenovic said Thursday there was a hole on Court 18 and that she and opponent Alison Riske had wanted to stop playing.

"The colour of the court, the fact that there's no more grass, the fact that the baseline where we are running, it's very slippery. There's no grass," said Mladenovic, who was beaten in three sets.

"There was a huge hole on the side. It was not even flat."

Swiss 19th seed Timea Bacsinszky had similar complaints after beating Olympic champion Monica Puig on the same court on Tuesday — just the second day of the even.

"I'm pretty disappointed about the quality of the grass, especially on Court 18. I'm not saying it's dangerous, but it was the second day of the tournament, and it was already ruined," said Bacsinszky.

"Sorry, Wimbledon, it's not against you, but there are improvements to do on this thing." 

Seven-time champion Roger Federer said that if both players complained about the state of a court then their concerns should be taken seriously.

"It's been extremely hot. You should always take the players' opinion serious, especially when both say it," said the Swiss.

The players found support from retired stars including Pam Shriver, who won five women's doubles titles at Wimbledon in the 1980s.

"The footing on grass when I played was good unless wet, but now they are rolling it so plays like a hard court, the footing will be worse," the American tweeted.

French 21st seed Caroline Garcia, who downed Madison Brengle on Court 18 on Friday, said the surface played differently depending on the time of day.

"My previous match I played over there on the fourth match, and I thought the court was very bad for second round.

Today at 11:30 was okay," Garcia said after making the third round.

Wimbledon courts are no stranger to injury controversies.

In 2013, Victoria Azarenka fell heavily, hurting her ankle and blamed the state of the courts. At the same event, Maria Sharapova said the surface on Court Two, where she lost to Michelle Larcher de Brito was "dangerous" after she fell a number of times