Marco Borriello gave the hosts an early lead but Siena turned the game around with goals either side of half-time from Innocent Emeghara and Alessandro Rosina, the latter from the penalty spot.
That put them on course for a crucial three points but Bosko Jankovic's equaliser dumped them back into the final relegation place, one point and one place below Genoa.
Siena coach Giuseppe Iachini had called on his side to play "the perfect game", but Borriello's sixth-minute opener was not in his script.
A free-kick was touched short by Juraj Kucka and Borriello's low shot took a deflection off the wall before nestling in the bottim corner.
Borriello turned provider for Kucka midway through a quiet first half, but the Slovakian fired just wide wide.
And Siena equalised two minutes before half-time when Massimo Paci nodded a corner down to the feet of Emeghara, who scored right-footed.
The second half was a slightly livelier affair, starting when Cesare Bovo pushed Alessio Sestu in the penalty area and Rosina scored from 12 yards.
Boriello twice went in search of an equaliser and went close when his diving header forced a sharp save from Pegolo. The striker's lob a minute later, though, was less accurate.
Jankovic levelled the scores in the 71st minute, though, slotting home low to the left of Pegolo after he was picked out in the penalty area by Ciro Immobile.
The same two players linked up in the 83rd minute, Jankovic crossing to Immobile, but this time the latter's header was brilliantly saved by Pegolo to preserve a point for Siena.