Former world number one Caroline Wozniacki retired from her first-round match at the Italian Open because of a leg injury.
But the 31st meeting between the most famous siblings in sport is now off after Serena Williams announced around 5.30pm yesterday that she was pulling out for the third consecutive tournament. Venus will now play fellow American Sloane Stephens in the third round.
Serena - now ranked 11th - has completed just eight matches this year, also withdrawing from Indian Wells in March with illness.
Williams took almost two months off after withdrawing ahead of the second round in Miami in March, also with a knee problem.
The withdrawal puts Williams' French Open in serious doubt, facing the possibility of having to play seven matches over two weeks if she wants to conquer Roland Garros.
"Obviously, I feel for Caroline, what she's going through with her injuries", said Collins.
She said: "I'll be concentrating on rehab and look forward to seeing you all at the French Open and next year in Rome".
She reached the fourth round before withdrawing ahead of a much-anticipated clash with Maria Sharapova due to a shoulder problem.
The French Open starts on 26 May.