Alessandro Mati was the hero as Juventus stunned an out-of-sorts Inter Milan 1-0 in Turin on Sunday to dent the champions' title hopes.
Inter now trail leaders AC Milan by eight points, with a game in hand, after falling to only their second defeat under Leonardo.
The champions produced a limp display against a dogged Juve, who breathed new life into their own Champions League hopes, moving up to sixth.
The previously in-form Inter started in confident fashion and carved Juve open after just two minutes.
However, with the option of two unmarked strikers to aim at, Wesley Sneijder tried to pick out Samuel Eto'o only to hit his pass straight at Frederik Sorensen, the ball spinning up and into the path of his other option, Giampaolo Pazzini, who scuffed his volley well off target.
On 14 minutes Juve almost took a fortuitous lead as Alberto Aquilani's overhit cross almost dropped under the bar.
In a half a few chances it was unsurprisingly a mistake that led to the opener.
Ivan Cordoba mistimed his leap to meet Sorensen's cross from the right, leaving Matri unmarked to head home from seven yards.
Inter fashioned an almost identical chance of their own on 50 minutes with Maicon centering for Pazzini but his header was straight at Gianluigi Buffon, who parried easily.
Despite having struggled throughout this season, Juve were giving as good as they got and Maicon had to be alert to block Matri when pulling the trigger from 10 yards before Giorgio Chiellini headed over from the resulting corner.
Sneijder had a sight of goal on 66 minutes but his fierce drive hit his own player, substitute Goran Pandev.
Pazzini followed that up with a volley that probably landed closer to the corner flag than the goal.
Inter's best chance came eight minutes from the end as Eto'o played a one-two with Sneijder, took the Dutch playmaker's lofted return but Buffon came out bravely to crowd the Cameroon captain's shot.
Juve almost punished Inter on the counter as Simone Pepe got down the left and crossed for Matri whose shot flashed past the near post.
A minute from time Inter knocked on the door again as Maicon surged forward and crossed from the right only for Eto'o at the back post to blast his shot from five yards onto the bar before moments later swinging at thin air when attempting a volley from an angle.
Earlier, Lazio kept their slim Serie A title hopes alive as they won 2-0 away to Brescia.
Edy Reja's team remain seven points off Milan, who thrashed Parma 4-0 on Saturday, and moved above Inter into third.
Goals from Alvaro Gonzalez on 18 minutes and Libor Kozak just before the hour mark ensured Lazio maintained their grip on a Champions Legaue place.
"We were top and then second and then we lost a few places but there are times throughout a long season in which it's normal to have a bad spell," said Reja.
Udinese are not letting up in their pursuit of a top four finish as they cruised to a 3-0 win at troubled Cesena.
Antonio Di Natale scored another brace, either side of a strike by Gokhan Inler, keeping fifth placed Udinese just two points behind third-placed Lazio.
Udinese coach Francesco Guidolin admitted it would be difficult to finish in the top four.
"Inter, Milan and Napoli are better than us so we have to beat all the others to finish fourth," he said.
"I think that's a difficult task but we'll see in a couple of months where we are."
It wasn't such happy news for Palermo, whose hopes of securing a Champions League finish suffered a blow as they fell to a shock 4-2 defeat at home to Fiorentina.