Roma's title hopes suffered a double blow yesterday when they were held 1-1 at home by Livorno and lost Francesco Totti to a serious knee injury.
Alessandro Diamanti snatched a useful point for relegation-threatened Livorno with a late free-kick after striker Mirko Vucinic had headed Roma in front.
In yesterday's other Serie A game, Fiorentina beat visitors Palermo 1-0 to move seven points clear of fifth-placed Milan and tighten their grip on the final Champions League qualifying spot.
Leaders Inter can extend their lead over Roma to six points if they win at Torino in today's late game. Four games will then remain in the title race, with Inter playing rivals Milan in two weeks' time.
Roma look set to be without injury-prone striker Totti for all of the run-in after the 31-year-old was substituted with a ligament injury in his right knee.
The club's medical staff are assessing whether Totti needs an operation, which media reports said could rule him out for up to five months.
Roma failed to create many clear-cut chances but broke the deadlock after 54 minutes when Montenegro's Vucinic netted a looping header after David Pizarro's clever chip into the area.
Livorno barely threatened but shocked the hosts seven minutes from time when substitute Diamanti curled the ball into the near corner of the net.
Livorno's Italy reserve goalkeeper Marco Amelia then saved twice from Daniele De Rossi, who was in tears at the final whistle as Roma, subject of a possible takeover bid, saw their chances of a first scudetto since 2001 become more remote.
Livorno remain in the red zone but are just a point behind fourth bottom Parma, who host Napoli today.
Fiorentina midfielder Marco Donadel grabbed the only goal with a 25-metre shot after 29 minutes to give them a boost ahead of Thursday's UEFA Cup semi-final, first leg at Rangers.
Inconsistent Palermo had Leandro Rinaudo sent off for a second booking midway through the second half but almost salvaged a draw when Mark Bresciano hit the post.
Playing Today - 15.00
Atalanta vs Juventus (0-1)
Cagliari vs Empoli (1-4)
Catania vs Lazio (0-2)
Milan vs Reggina (1-0)
Parma vs Napoli (0-1)
Sampdoria vs Udinese (2-3)
Siena vs Genoa (3-1)
Torino vs Inter (0-4) 20.30
Next fixtures
Sunday: 15.00 Fiorentina vs Sampdoria; Genoa vs Empoli; Inter vs Cagliari; Juventus vs Lazio; Livorno vs Milan; Napoli vs Siena; Palermo vs Atalanta; Reggina vs Parma; Roma vs Torino; Udinese vs Catania.