Napoli defender Kalidou Koulibaly scored an injury-time own goal to hand Juventus a 4-3 Serie A victory in Turin on Saturday after the visitors had fought back from three goals down.
Goals from Danilo, Gonzalo Higuain and Cristiano Ronaldo gave the Italian champions a 3-0 lead with less than half an hour remaining.
Napoli refused to give up as Kostas Manolas and Hirving Lozano scored within two minutes of each other before Giovanni Di Lorenzo completed a stunning comeback.
But Koulibaly volleyed a free kick into his own net in stoppage time to hand Juve an early advantage in the Serie A title race and leave Napoli coach Carlo Ancelotti ruing a missed opportunity.
"Juventus have so much character and quality, we know that they always step up in the crunch moments of the season," he told Sky Sport Italia.
"We can compete, but we wasted an opportunity today, above all that hour where we just didn’t play. That is really disappointing."
Meanwhile, A C Milan coach Marco Giampaolo admitted he still has much work to do after earning his first win at the club as Hakan Calhanoglu struck early to clinch a 1-0 home victory over Brescia in match another Serie A on Saturday.
The 18-times Italian champions failed to muster a single shot on target when they fell to a shock 1-0 defeat away to Udinese on the opening weekend.
Yet Calhanoglu’s headed goal after 12 minutes settled the nerves of the San Siro crowd and proved to be decisive in a cagey encounter.
Juventus coach Maurizio Sarri was denied a reunion with his former club as he sat in a different section of the stadium to aid his recovery from pneumonia, but he would have been pleased with his team's performance in the first half.
Substitute Danilo needed just 26 seconds to score after replacing the injured Mattia De Sciglio, finishing off a flowing counter-attack led by Douglas Costa.
Within three minutes Higuain made it 2-0 after he fired into the top corner to mark his third goal in six league games against his former employers, before Sami Khedira curled a shot off the cross bar.
Ancelotti reacted at the break by introducing Mario Rui and debutant Hirving Lozano, but the game appeared to be over as a contest just after the hour mark when Ronaldo opened his account for the season with a well-taken finish.
The match turned on its head in the space of three minutes when Manolas pulled one back with a header before substitute Lozano converted Piotr Zielinski’s cross to reduce the deficit to one goal with more than 20 minutes remaining.
Napoli goalkeeper Alex Meret produced an outstanding save to tip a Costa strike onto the bar but the momentum was with the visitors and Di Lorenzo got in front of Juventus debutant Matthijs De Ligt to bundle in from a free kick and complete the comeback.
But the game was lost in heartbreaking fashion when Koulibaly diverted Miralem Pjanic’s free kick into his own net in the 92nd minute.
Barca held to surprise draw at Osasuna
Sixteen-year-old Ansu Fati became Barcelona's youngest ever La Liga goalscorer on Saturday but could not prevent his side stumbling to a surprise 2-2 draw at promoted Osasuna which left the champions with only four points after three games.
Roberto Torres's thumping volley gave Osasuna the lead in the seventh minute to send the boisterous El Sadar crowd into a frenzy and Barca struggled to cope with the home side in a first half in which they failed to have a single shot at goal.
Barca were still without all-time top scorer Lionel Messi and fellow forwards Luis Suarez and Ousmane Dembele due to injury and coach Ernesto Valverde was left with little choice but to turn to Fati at the start of the second period.
The teenager had become Barca's second-youngest ever player in last week's 5-2 win at Real Betis and he needed only six minutes to get off the mark, rising to head home a cross from fellow academy graduate Carles Perez.
Perez also provided the pass which led to Arthur Melo giving Barca the lead in the 64th minute by brushing off two defenders inside the area before delivering a superb strike into the bottom corner.
But even when they were in front, Barca looked far from comfortable against an Osasuna side renowned for their physical prowess and who are unbeaten at El Sadar in league games since April 2018.
Forward Brandon should have equalised for the hosts when he was unmarked in the area and got his head on a cross but missed the target, while substitute Juan Villar was thwarted by Barca goalkeeper Marc-Andre ter Stegen a minute after coming on.
Osasuna finally found their way back into the game when Gerard Pique was penalised for blocking a cross with his hand and Torres showed nerves of steal to send Ter Stegen the wrong way from the spot to level in the 81st minute.
Bayern fire six past hapless Mainz
Bayern Munich scored six times in a span of 45 minutes to come from a goal down and crush visitors Mainz 05 6-1 on Saturday and climb to seven points from three games, two off the top.
After an unexpected home draw on the opening match day Bayern have now won their last two matches convincingly, scoring nine goals and moving up to seven points.
RB Leipzig are top on nine points after their 3-1 win at Borussia Moenchengladbach on Friday.
Borussia Dortmund slumped to a shock 3-1 loss at promoted Union Berlin to remain on six points.
Benjamin Pavard's fine volley in the 36th minute cancelled out Mainz's lead through a Jean-Paul Boetius header and launched Bayern's sustained attack.
A sensational David Alaba free kick on the stroke of halftime turned the game around before Ivan Perisic gave Bayern a two-goal cushion with a header at the far post in the 54th.
Kingsley Coman stabbed in their fourth 10 minutes later and league top scorer Robert Lewandowski also got onto the score-sheet, firing in his sixth goal in three league matches in the 78th with Mainz' defence in tatters.
Substitute Alphonso Davies completed the rout two minutes later.
"Initially we were too sleepy. We did not counter Mainz's aggressive start and we thought it would be easy," said Bayern coach Niko Kovac.
"Then we played the ball faster and played well over the wings. The 2-1 by Alaba before the break was key. We are going into the two-week international break happy."
The Bundesliga resumes on September 13 after a break for Euro 2020 qualifiers.
Schalke 04 scored their first win of the season after two own goals from Hertha Berlin helped them to a 3-0 win.
Cologne came from a goal down to beat Freiburg 2-1 for their first win of the campaign.
VfL Wolfsburg dropped their first points with a 1-1 draw against promoted Paderborn and are level with Bayern on seven.