ROME-Serena Williams has withdrawn from the Italian Open ahead of her second-round match against sister Venus due to an old left knee injury.
Four-time Rome victor Serena, 37, had eased through 6-4, 6-2 her first-round match against Swedish qualifier Petersen.
It's the third straight event that the 37-year-old Serena has exited from due to physical problems. Venus will now play fellow American Sloane Stephens in the third round. "I will miss the fans and competition at one of my favorite tournaments".
Serena had also played Petersen in the second round at the Miami Open in March before withdrawing with a knee injury.
American Collins, 25, will now play two-time Grand Slam victor Garbine Muguruza of Spain in the second round of the Italian Open on Wednesday following Wozniacki's withdrawal.
Williams took almost two months off after withdrawing ahead of the second round in Miami in March, also with a knee problem.
She is a three-time French Open victor, claiming her last title in Roland Garros in 2015.
Williams said she has been dealing with a knee injury.
Wozniacki is ranked 12th in the world and will be hoping to be fit for the French Open, which starts on 26 May.