Sio scored late on to deny Steaua Bucharest a rare Champions League win at St Jakob Park.
In-form striker Federico Piovaccari scored his first Champions League goal on 17 minutes to fire Steaua ahead and they looked set to end a 20-game winless run in the competition as time ticked down.
But Sio broke the away side's hearts with the latest of equalisers and almost certainly cut Steaua out of the running to progress from Group E.
Instead, Basel edged a point behind second-placed Schalke, who were beaten 3-0 at Chelsea.
Basel were gifted an early chance when Lukas Szukala was whistled for handball at the edge of the area before Matias Delgado fizzed the free-kick just wide.
The home side had the best of the early exchanges but Steaua took the lead on 17 minutes when Piovaccari punished them for setting their defensive line far too high up the pitch.
The problem was compounded by the lack of pressure on Szukala who had time to loft a 30-yard pass over Basel's static central defenders, who watched on as Piovaccari ran by.
The striker therefore had ample time to take a touch and almost stop before waiting for Yann Sommer to commit himself. When the goalkeeper eventually dived left, Piovaccari arrowed his shot the other way.
Basel's response was for Fabian Frei to fire a 12-yard shot on the turn narrowly over shortly after.
Ciprian Tatarusanu then denied Mohamed Saleh one-on-one as Basel's best chance of the half went to waste.
Steaua coach Laurentiu Reghecampf made a change at the break with Lucian Filip replacing the injured Mihai Pintilii.
The away side had looked dangerous on the break before half-time and sat deep in the second half as they sought to hold on to their lead.
A three-on-three break was snuffed out by some last-gasp defending by the home side whose best chances were limited to set-pieces.
Ivan Ivanov flashed a header past the post from a corner while Valentin Stocker saw an effort blocked away.
The few Steaua fans who had made the trip looked set to celebrate a rare win only for Sio to pop up at the end with a memorable strike.
Stocker played a perfect pass for the substitute whose first time shot beat the goalkeeper and earn Basel a precious point.