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After slipping on the grass on Centre Court, 24-time Grand Slam champion Serena Williams was forced to retire Tuesday with an injury.

The star later took to Instagram to express her gratitude to the fans for their support.

“I was heartbroken to have to withdraw today after injuring my right leg,” Williams wrote. “Feeling the extraordinary warmth and support of the crowd today when I walked on — and off — the court meant the world to me.”

Williams’ fall has raised questions about the safety of the courts. Adrian Mannarino also had to retire after slipping during the previous match on Centre Court against Roger Federer.

Two-time Wimbledon champion Andy Murray, who played on Centre Court on Monday, tweeted that it’s not easy to move on the surface of the tournament’s main stadium.

“It feels a tad more slippery maybe under the roof, I don’t know if it’s a gut feel,” Federer told reporters after learning of Williams’ injury during his press conference.

“You do have to move very, very carefully out there, and if you push too hard in the wrong moments, you do go down … this is obviously terrible. I don’t think it plays very different, but again I’m also moving carefully.”

Williams’ opponent, Aliaksandra Sasnovich, echoed this in her post-match press conference. She said she wasn’t running for shots as she normally would out of caution.

“It was very slippery — I fell as well,” she said. “When she did an angle, I couldn’t run, because it was so slippery.”

The All England Lawn Tennis Club responded in a statement to CNN following the incidents to defend the conditions of the grass.

“The preparation of the grass courts has been to exactly the same meticulous standard as in previous years,” the organization said. “The weather conditions on the opening two days have been the wettest we have experienced in almost a decade, which has required the roof to be closed on Centre Court and No.1 Court for long periods. 

“This is at a time when the grass plant is at its most lush and green, which does result in additional moisture on what is a natural surface.”