Juventus' hopes of clinching the Serie A title were put on hold following a 2-1 loss at Udinese on Thursday evening.
Paulo Dybala captained the league leaders, who missed Leonardo Bonucci through suspension while Giorgio Chiellini sat out again through injury.
Juventus lived dangerously in the opening six minutes when Danilo's glancing header cracked the post after looking to clear a cross from the left-hand side.
Udinese shot-stopp Juan Musso got to work for the first time after going across his area to palm Dybala's first-time strike from outside the area for a corner-kick 13 minutes into the match.
Ronaldo almost rattled the top corner from the edge of the area a minute to the waterbreak after twisting and turning before going narrowly wide with a right-footed attempt.
The Old Lady hit the front through Matthijs De Ligt three minutes before halftime as the Dutchman drilled home a loose ball from 25 yards out, giving Musso no chance.
Juventus were given an early scare two minutes after the break when a loss of focus from Danilo allowed Ilija Nestorovski in with a one-on-one chance, but Wojciech Szczesny saved.
Nestorovski wasn't to be denied by the Juve shot-stopper minutes later after barging in at the back-post to head into the far corner following a cross by Ken Sema.
An alteration from the visitors arrived as Douglas Costa and Blaise Matuidi set in, with Aaron Ramsey and Federico Bernardeschi making way six minutes thereafter.
Dybala earnestly pushed to restore their advantage as the final quarter approached, but Musso stupendously plugged his curler from the top corner.
Juve were dealt a blow when Seko Fofana landed onto a loose ball before going on to beat two challengers for a composed finish in stoppage time to hand Udinese a famous while halting the visitors' title celebrations for now.
Lazio 2-1 Cagliari
Lazio have officially clinched a UEFA Champions League spot in Serie A after defeating Cagliari 2-1 on Thursday evening.
Joao Pedro put Cagliairi ahead inside two minutes with a stunning free-kick that flew into the top corner, but it was disallowed by the referee Marco Piccinini.
Manuel Lazzari launched the home side's first opening after lashing in a first-time volley at the back-post, which Alessio Cragno turned behind on ten minutes.
Cragno's workload didn't stop there after denying Ciro Immobile his 31st league goal of the season with a smart palm as the half-hour mark approached.
Walter Zenga's boys nabbed the breakthrough on the stroke of halftime after Giovanni Simeone's close-range strike wrong-footed Thomas Strakosha before slipping in.
Lazio levelled matters with a stunning first-time volley from Sergej Milinkovic-Savic two minutes after the restart, leaving Cragno, who up till then had a splendid evening, stunned.
Immobile was denied again as Cragno pushed his shot-on-the-turn onto the left-hand post 55 minutes in following a cross out wide.
The Lazio striker finally hit the jackpot after running onto the shoulder of the danger area to slot into the bottom corner 60 minutes gone following Alberto's through ball.
Cagliari weren't able to launch a reply following the closing phases, while the Eagles added more firepower from the bench in Joaquin Correa.
Little pressure from the Rossoblu of Sardegna arrived as the contest began fizzling out, with the final score reading 2-1 to Lazio who ends a five-match winless run to secure a top-four spot.