Adam Marusic fired Lazio to late victory over Juventus with his stoppage-time header in Saturday's 1-0 win in Rome, as a 3-0 home humbling at the hands of Atalanta left Napoli's Champions League hopes hanging by a thread.
Marusic thumped home Matteo Guendouzi's perfect inswinging cross in the third minute of added time at the Stadio Olimpico to ensure victory in Igor Tudor's first match as Lazio coach.
The Montenegro defender's first goal of the season pushed Lazio up to seventh, a point and a place above Napoli.
Lazio are eight points away from the last Champions League spot currently held by fourth-placed Bologna ahead of their Easter Monday fixture with Salernitana.
"You can't wish for a better start. I'm happy for the boys as they couldn't have played any better given the time we had together," said Tudor to DAZN.
Juve, in third, are stuck firmly in a rut after defeat in the Italian capital left them four matches without a win and sliding back towards a battle for a Champions League spot.
Massimiliano Allegri's side have picked up just seven points from their last nine games and are eight points ahead of fifth-placed Roma who face Lecce on Monday.
"We have to believe that we'll achieve our goal because we have a few points on the other teams," said Allegri to DAZN.
"You can't concede goals like the one we did with 10 seconds remaining. We're as annoyed by the way we lost the game as much as we are at losing it."
Juve were without suspended Dusan Vlahovic and fellow attackers Arkadiusz Milik and Filip Kostic, and lacked inspiration throughout a scrappy contest in Rome.
Lazio made life tough for Juve in Tudor's debut in the dugout after taking over from Maurizio Sarri as permanent boss at the Roman outfit.
And he was submerged in hugs on the touchline when Marusic decided the contest to give him the perfect start to life at Lazio.
Tudor, who won Serie A and B titles over nine years as a Juventus player, will be encouraged by the solid display put on by his team.
The former Croatia defender has a tough opening week at Lazio as his side face Juve again on Tuesday, in Turin for the first leg of their Italian Cup semi-final, before the Rome derby next weekend.
Napoli are way behind Bologna after first-half goals from Aleksei Miranchuk and Gianluca Scamacca and a late Teun Koopmeiners strike gave Atalanta a convincing win in Naples.
Francesco Calzona's side have almost no chance of a place in next season's revamped Champions League unless Italy get an extra spot as they are six points off Roma.
"We're not mathematically out of it but the chances of us getting there are very slim," said Calzona after his first league defeat as Napoli coach.
Both Roma and Bologna have to travel south for matches at the Stadio Diego Armando Maradona before the end of the campaign, while Atalanta lurk a point behind Roma with a game in hand.
Scamacca's low strike was his first in the league since the end of January but his third in four matches after netting in both legs of Atalanta's Europa League win over Sporting Lisbon.
The 25-year-old is hoping for a spot in Italy's Euro 2024 squad after missing out on the European champions' most recent friendlies.
"(Italy coach Luciano Spalletti) can always count on me because I always give my best... I'm more consistent now and I'm very happy, above all for Atalanta who showed faith in me," said Scamacca.
Napoli's players took the knee in support of their teammate Juan Jesus during the pre-match playing of the Serie A anthem while fans shouted "no to racism" from the stands.
Jesus and Napoli were stunned by the Italian Football Federation's disciplinary tribunal clearing Inter Milan defender Francesco Acerbi of racially abusing Jesus in a game two weeks ago for lack of evidence.
