Gonzalo Rodriguez's goal was enough for the Viola to keep alive their faint glimmer of a chance of qualifying for the Champions League as they go into the final two games of the season four points adrift of third-placed Milan.
The win also practically guarantees them a place in the Europa League while Siena now need a miracle to stay up with six points separating them from Genoa with two games of the season remaining.
The visitors, aware of Milan's win over Pescara before kick-off, started brightly with Juan Cuadrado sending in a warning shot which missed the target in the third minute.
Just 10 minutes passed before Vincenzo Montella's men took the lead.
Pasqual's low cross from the right was bundled over the line by Rodriguez, who used his stomach to ensure the ball went in.
Fiorentina nearly doubled their lead 10 minutes before the interval when Borja Valero's shot was pushed onto the post by Gianluca Pegolo.
Siena were unable to muster any kind of reaction and Fiorentina continued to look closer to a second than their hosts were to an equaliser.
Matias Fernandez flashed a shot just over the crossbar before Giulio Migliaccio did the same with a header.
Emiliano Viviano was finally called into action in the 70th minute to push Alessandro Rosina's shot away, but it was too little from a Siena side who needed points to stand any chance of staying up.
Fiorentina substitute Luca Toni also hit the crossbar as Fiorentina eased to three points which keeps them breathing down Milan's necks, but more importantly, gives them a practically guaranteed place in Europe next season.