Twenty-three-time Grand Slam champion Serena Williams has pulled out from the Italian Open due to a knee injury.

The 37-year-old American was set to face her sister,  Venus Williams in the second round but said she would be “concentrating on rehab” now.

She said:

“I look forward to seeing you all at the French Open and next year in Rome,”

Meanwhile, Former world number one Caroline Wozniacki hashalso pulled out in Italy.

28-year-old Wozniacki withdrew from her first-round match against American Danielle Collins because of a leg injury, having lost the first set 7-6 (7-5).

It is the second straight tournament at which the Dane has been forced to retire early on.

American Collins, 25, will now play two-time Grand Slam winner, Garbine Muguruza of Spain in the second round of the Italian Open on Wednesday following Wozniacki’s withdrawal. Speaking on this, Collins said:

“Obviously, I feel for Caroline, what she’s going through with her injuries,”

“It’s never fun to go out there and play against somebody that’s hurt, and dealing with that kind of pain. I was happy with the way I played, but it’s just not a fun situation.”

The tennis stars’ focus is now on the French Open tournament which kicks off on 26 May. Serena Williams will be chasing her fourth title, while Wozniacki, a former Australian Open champion, has never gone beyond the quarter-final stage.