Three goals in the opening 10 minutes from Alessandro Del Piero (2) and Antonio Candreva looked to have secured the points for Alberto Zaccheroni's fifth-placed side, who had entered the match on the back of three wins from their last four games.But former Middlesbrough striker Massimo Maccarone got a goal back for the basement battlers just before the interval before two goals from Abdel Ghezzal, the second from the spot, sealed the most unlikely of comebacks.
The result will boost Siena's hopes of avoiding the drop as it means they are four points adrift of safety having been unbeaten in five Serie A matches.
It would have been ludicrous to predict such an outcome after Juve came out of the blocks in devastating fashion by taking advantage of some poor Siena defending.
There were two minutes on the clock when veteran forward Del Piero grabbed his first goal.
Mohamed Sissoko's shot was saved by Gianluca Curci, but the goalkeeper had no hope of keeping out Del Piero's effort as he lay prone on the ground.
Del Piero then doubled the lead after seven minutes to claim his 300th career goal for the Old Lady.
The goal again came from a rebound after Claudio Marchisio's effort was beaten out by Curci and the ball fell to Del Piero, who struck a spectacular half-volley into the net.
The game appeared to be over as a contest when France international David Trezeguet teed up Candreva for a 10th-minute strike to make it 3-0.
But Siena showed great character to begin their fightback six minutes later through Maccarone.
A lapse in defence by Zdenek Grygera saw the ball reach the striker, who made no mistake with a powerful right-footed effort.
The plucky visitors continued to push forward and came close to closing the deficit further in the 21st minute.
A great cross picked out Francesco Pratali, but his header was cleared off the line by Juve's Felipe Melo.
But Alberto Malesani's side were not to be denied and pulled another goal back shortly after the start of the second half.
Maccarone hit the post after good build-up work, but Ghezzal was on hand to tap in the rebound.
Juve were stunned into action and Del Piero went agonisingly close to his hat-trick, but a magnificent save by Curci saw the goalkeeper claw out his close-range header.
The visitors were by now carving out chances with alarming regularity and it took a superb last-ditch clearance from Fabio Cannavaro, after Marcelo Larrondo's cross, to prevent an equaliser.
Alexandros Tziolis then hit the post for the Robur, but the leveller finally came in the 73rd minute.
Grygera's poor afternoon continued as he fouled Maccarone and the referee pointed to the spot.
And Ghezzal made no mistake to heap further woe on the home side.
SIENA Comeback Jolts JUVE
