The Turin faithful was silenced by the forward who stole in during the dying minutes to level proceedings, putting the capital club temporarily ahead by virtue of away goals
Federico Peluso had put the Old Lady ahead in a game where they outplayed the visitors, but Antonio Conte's men will have to chase a result in the return fixture to survive in the competition.
Chasing a place in the Coppa Italia final to keep the dream of a third domestic double on track, Juventus made an explosive start to the match in front of their own fans. The match was barely a minute old when the home side broke forward, but Claudio Marchisio saw the ball robbed from his grasp by goalkeeper Federico Marchetti after being played in by Alessandro Matri. Leonardo Bonucci also went close early on with a header that went just over the bar, while Paul Pogba also threatened in the air.
Lazio were pinned back for the first 20 minutes, but gradually began to fight in a fixture that became progressively scrappier and more imprecise. Their best chance of the first half fell to Mauri; the forward had the chance to poke home when Bonucci scuffed a clearing header, but hesitated and saw the ball cleared by Luca Marrone with the net wide open.
Both sides had started the first leg with six midfielders, and the result was a messy encounter that frequently became bogged down around the centre circle. Passes were going wayward across the pitch, with Lazio's creative talents Mauri and Hernanes particularly notable for their lack of calm with the ball at feet.
Juventus fans would hold their breath once more in the opening 45 minutes when Emanuele Giaccherini released a stinging effort from deep as the half entered injury time. Marchetti was more than equal to his shot, however, and the teams entered the tunnel still deadlocked.
Despite the rather turgid nature of play neither team made changes during the interval, although Juve had already been forced to bring Martin Caceres into the action following an injury to Bonucci. The Turin side switched the focus of attack, looking to put pressure on their rivals out wide through the exertions of Mauricio Isla, but Lazio were resolute and resisted a host of crosses fired in by the Chilean.
Nine minutes into the second half, Juventus were presented with their best chance of the match. Matri broke the offside trap and did well to latch on to a stunning lofted pass from Marrone, taking his shot first time on the volley. The effort lacked power, however, and Marchetti again did well to palm away and keep his team level with the hosts. Mauri then appeared with a header at the other end, but Marco Storari was also in the right place and saved comfortably.
Pressure on the lacklustre Romans was growing, and it was no surprise that, after 63 minutes, the Old Lady pushed themselves into the lead. A fluid passing move around the box left Giaccherini in space, and his pinpoint cross found Federico Peluso who nodded the ball into the corner of the net. Lazio were furious that a supposed foul in the area had not been called, but it did not matter; the goal stood, and Juve were in the driving seat.
Arturo Vidal had been strong in midfield throughout the clash, and the Chilean went agonisingly close to extending his team's advantage when his well-placed shot beat Marchetti, but struck the post, and from the rebound, the midfielder steered Barzagli's cross inches wide to give Lazio a lifeline. The Aquile took full advantage, as five minutes from the end Mauri stole in at the far post to poke home a corner and hit a shock equaliser.
Juventus will now have to score in Stadio Olimpico to progress from the last-four clash. The second fixture is scheduled to take place on January 29, while Roma and Inter play their opening encounter in the Italian capital on Wednesday.