Roger Federer and top-ranked Naomi Osaka withdrew before their Italian Open quarterfinals because of injuries on Friday.
Federer reported a right leg injury ahead of his match against Stefanos Tsitsipas, while Osaka said her right hand was injured before she was to play Kiki Bertens.
Federer and Osaka both won two matches on Thursday after play was backed up due to rain a day earlier.
While Osaka won both of her matches in straight sets, the 37-year-old Federer had to labour for more than 2 ½ hours to overcome Borna Coric in his second time on the court.
"I am disappointed that I will not be able to compete today. I am not 100 per cent physically and, after consultation with my team, it was determined that I not play," Federer said. "Rome has always been one of my favourite cities to visit and I hope to be back next year."
It's only the fourth time in Federer's career he has had a walkover loss, the ATP Tour said, adding the 20-time Grand Slam champion has never retired in 1,465 matches.
Federer was not originally planning to play in Rome but he changed his schedule last week, saying he would rather play matches than practice ahead of the French Open, which starts in nine days.
Tsitsipas and Bertens advanced to the semifinals via walkover.
Osaka couldn't immediately say how serious the injury was, or if it will affect her status for Roland Garros. She was yet to see a doctor but when she held her hand up for reporters to see it was clearly swollen.
"I woke up this morning and couldn't really move my thumb," Osaka said. "I tried to practice and grip my racket but I couldn't, and I kept feeling this pain when I tried to move my hand in different directions."
Osaka's win on Thursday guaranteed that she will remain No. 1 going into the French Open.
"I didn't feel anything yesterday. That's why I'm kind of confused right now because I literally woke up in the morning and couldn't move my thumb," Osaka said. "So I was like, 'Maybe I slept on it and maybe it will go away.' But it didn't."
Bertens, who won the Madrid Open last week, will face Marketa Vondrousova or Johanna Konta for a spot in the final.
The quarterfinals in the other half of the draw feature former No. 1 Victoria Azarenka against Karolina Pliskova, and Kristina Mladenovic against Maria Sakkari in a matchup of two qualifiers.
Tsitsipas' semifinal opponent will be eight-time Rome champion Rafael Nadal or Fernando Verdasco.
The other men's quarterfinals are top-ranked Novak Djokovic against Juan Martin del Potro, and Kei Nishikori against Diego Schwartzman.
Osaka also withdrew before a semifinal in Stuttgart, Germany, last month due to an abdominal injury. And she retired from her previous meeting with Bertens at last year's WTA Finals with a leg injury.
"I feel like the ab thing could have been helped, but this one I don't think I could have helped it because I don't know what caused it," Osaka said. "I don't know why I have it."