Germany's Tommy Haas, seeded 15, retired from the Rome Masters after losing the first set 6-2 in his quarter-final with Bulgarian opponent Grigor Dimitrov on Friday.
Haas, who appeared to be suffering from injury, lasted just 35 minutes before informing officials he could no longer carry on and shaking Dimitrov's hand before leaving the Central Court at the Foro Italico.
Dimitrov, seeded 12, has advanced to the semis and will now face the winner of Friday's later quarter-final between top seed Rafael Nadal and Andy Murray, seeded seventh, for a place in the final.
Earlier, Canadian eighth seed Milos Raonic moved into the semis with a commanding 6-3, 5-7, 6-2 quarter-final win over Frenchman Jeremy Chardy on Friday.
Raonic, considered an outside bet for the French Open later this month, will now meet the winner of the quarter-final clash between second seed Novak Djokovic and David Ferrer, who is seeded fifth.