Roddick was doubled up in pain after suffering his back strain
Novak Djokovic will face Stanislas Wawrinka in the Rome Masters final after both their semi-final opponents retired through injury on Saturday.
Roger Federer's conqueror Radek Stepanek bowed out to Djokovic while trailing 6-0 1-0.
And Andy Roddick withdrew against Wawrinka with a back injury when trailing 3-0 in the first set.
The event has been dogged by injuries with Nicolas Almagro pulling out against Djokovic in the previous round.
Unseeded Swiss Wawrinka was leading 3-0 when Roddick pulled up awkwardly after his opponent struck a forehand winner.
The American, 25, received courtside treatment but was unable to continue, handing his opponent an easy passage into his second final of the year.
"I felt a little something last night in my back when I was getting treatment. Then, one wrong movement and I had a complete spasm," Roddick said.
"I warmed up OK, but then just one movement triggered it. I can feel it even when I take a deep breath now
"I can't really move my left arm right now. You can't really play around that."
Stepanek received lengthy treatment at the end of the first set, but managed just one more game before retiring.
"I started feeling bad for the first time after practice this morning and then I ate lunch and was feeling better," he said.
"Once the match started, however, I started feeling worse and worse. I was dizzy and weak and couldn't play."
Wawrinka will be aiming to repeat the success of Spain's Felix Mantilla, who defeated Federer in the 2003 final to become the only unseeded Rome champion of the Open era.