Juventus Stadium was silenced by the forward who tallied late to put Lazio temporarily ahead by virtue of away goals in the cup semifinal.
Federico Peluso had put Juventus ahead in the 63rd minute, but Antonio Conte's men will have to chase a result in the return fixture to survive in the competition, with Lazio leading by virtue of the away goal tiebreaker.
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 its 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.
Lazio was pinned back even more for the first 20 minutes, but it gradually began to fight in a fixture that became progressively scrappier and more imprecise. The visitors' best chance of the first half fell to Mauri, as the forward had the chance to poke home when Leonardo Bonucci scuffed a clearing header, but he hesitated and saw the ball cleared by Luca Marrone with the net wide open.
Nine minutes into the second half, Juventus was presented with its best chance to that point. 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 lackluster Romans was growing, and it was no surprise Juve pulled in front. A fluid passing move around the box left Emanuele Giaccherini in space, and his pinpoint cross found Peluso, who nodded the ball into the corner of the net.
Lazio endured, though, and Arturo Vidal, who had been strong in midfield throughout the clash, was key in turning the tide. The Chilean's well-placed shot beat Marchetti only to be denied by 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 equalizer.
Juventus will now have to score in Stadio Olimpico to advance to the final. The second fixture is scheduled to take place on Jan. 29, while Roma and Inter play their opening leg of the semifinals in the Italian capital on Wednesday.