Federico Peluso's first goal for Juventus looked to have been enough for the home side until Mauri met a corner unmarked at the far post to ensure Antonio Conte's side have it all to do in next week's return leg.
Below, Sports Mole looks back on a frustrating 90 minutes for the reigning Serie A champions.
Was the result fair?
Certainly not. Lazio will know they got out of jail this evening after profiting from a wasteful Juventus attack. The home side dominated possession and on another night would have run out comfortable winners. Juve pressed the action throughout, particularly so in the second half, and can feel genuinely hard done by not to win tonight's match.
The Old Lady controlled possession and looked a constant threat in the final third. A more clinical striker than Alessandro Matri would have put Juve within touching distance of the final early in the second half. Paul Pogba continued his impressive recent form with an all-action display in the early stages before fading late on.
A spirited defensive display was rewarded against the run of play four minutes of time. Hernanes has been superb this season but Lazio's creative spark could not find a way into tonight's match. Vladimir Petkovic threw on two strikers in the closing stages and his gamble earned the visitors a vital away goal.
Sports Mole's man of the match
Federico Marchetti: The Lazio keeper produced a string of magnificent saves to keep his side in the tie. His best stop by some distance was a miraculous late dive to deny Arturo Vidal's header, which was seconds from crossing the line.
Second-half substitute Martin Caceres failed to mark Mauri at the back post from Lazio's only attack of the second half and his late lapse in concentration proved costly.
Antonio Damato enjoyed a quiet evening and had to do very little during a keenly-contested encounter. His one minor indiscretion came in stoppage time when Michael Ciani was booked for little more than standing next to Mirko Vucinic.
Juventus: Conte's men continue their quest to retain Serie A on Saturday with a home match against struggling Genoa.
Lazio: Petkovic's side return their focus to a push for Champions League places at the Olimpico against Chievo.