Twenty-three time Grand Slam champion Serena Williams has withdrawn from the Italian Open with a left knee injury.

The American, who also pulled out of the Miami Open at the third-round stage in March, was set to play sister Venus Williams in the second round in Rome.

The 37-year-old has only completed eight matches in 2019 and was forced to pull out of Indian Wells because of a viral illness.

Former world number one Caroline Wozniacki has also withdrawn.

The Dane retired from her first-round match because of a leg injury after she lost the opening set 7-6 (7-5) against American Danielle Collins.

It is the second straight tournament at which the 28-year-old has withdrawn early on.

She trailed 3-0 against Alize Cornet in the opening round of the Madrid Open earlier this month and had to pull out because of a back injury.

Collins, 25, will now play two-time Grand Slam winner Garbine Muguruza of Spain in the second round on Wednesday.

"Obviously, I feel for Caroline, what she's going through with her injuries," said Collins.

"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."

Both players will hope to be fit for the French Open, which starts on 26 May.

Former Australian Open champion Wozniacki, ranked 12th in the world, has never gone beyond the quarter-finals at Roland Garros.

Three-time champion Williams last won the clay tournament in 2015 but was runner-up the following year