The visitors twice came from behind to deny Roberto Mancini's side, who would have lifted their third consecutive Scudetto had they claimed victory.
Former Arsenal midfielder Patrick Vieira headed in from close range to give Inter the lead before ex-Middlesbrough striker Massimo Maccarone equalised on the hour mark. Mario Balotelli restored Inter's advantage just before the break, only for Siena to again draw level through Houssine Kharja's long-range strike after 70 minutes.
Roma's 2-1 win at home to Atalanta ensured the race for the title will go down to the final weekend. Christian Panucci and Daniele De Rossi scored for the Giallorossi.
Fiorentina moved back up to fourth spot to boost their hopes of qualifying for the Champions League with a 3-1 victory over 10-man Parma.
The result sees the Florence outfit move two points clear of AC Milan, who lost 3-1 at Napoli. Milan's hopes of clinching the last Champions League spot were dealt a blow with Marek Hamsik, Maurizio Domizzi and Gyorgy Garics earning victory for Napoli.
Cagliari secured their survival as they defeated high-flying Udinese 2-0, and Reggina also made sure they would be playing Serie A football next term with a 2-0 home win over Empoli.
A last-gasp Alessandro Del Piero strike earned Juve a point and denied visiting Catania the chance to secure their safety. Jorge Martinez fired the relegation-threatened side ahead three minutes into the second half but Del Piero, whose side have already secured third spot, pounced in the dying seconds.
Lazio salvaged a touch of pride from a disappointing season by winning 2-0 at Genoa, condemning their hosts to a third consecutive league defeat.
And Sampdoria triumphed 2-0 at Palermo to continue their push for top-six finish, despite being guaranteed a UEFA Cup spot next season thanks to Roma's qualification for the Coppa Italia final in midweek.