PSG looked beaten until Di Maria curled home in the third minute of time added on, salvaging a point from a game in which they had been second-best for long spells.
Prior to that it looked as though goals from Lorenzo Insigne and Dries Mertens, either side of a Mario Rui own goal, would give Carlo Ancelotti’s Napoli the victory in the French capital.
Instead, the biggest beneficiaries of this result are surely Liverpool, who move to the top of Group C after easing to a 4-0 win over Red Star Belgrade in the night’s other game.
PSG are third, a point behind Napoli with a trip to Italy up next before Liverpool visit Paris next month. That means there is a real risk that a club with ambitions of winning the Champions League could be sent packing in the group stage.
“We can improve, it’s not possible to make errors because the opposition are very strong,” said Tuchel, who has been showered with praise with his side winning all 10 of their games in Ligue 1, a record-breaking start to a French season.
“I have lots of respect for the teams in this group. I knew before it would be really complicated, and in my opinion we are not the favourites. This group is really difficult and we can’t be surprised.”
Doubts had already been raised about PSG’s true capabilities at this level when they performed poorly in losing 3-2 to Liverpool at Anfield, and once again they were exposed here, with Ancelotti revelling in his latest return to Paris.
It was an accomplished away performance from Napoli, but PSG’s all-star attack of Neymar, Kylian Mbappe and Edinson Cavani toiled for much of the evening, which was a world away from the strolls they so often enjoy domestically.
“It is always complicated against an Italian team because of the culture they have,” said Tuchel.
“We are not an Italian side, we are not Atletico Madrid, we are PSG and we have another style.
“They have been together for years, and we have been together for just 11 weeks, but we must improve, that is clear.”
– Napoli ‘deserved victory’ –
Ancelotti coached PSG to the Ligue 1 title in 2013 before leaving for Madrid, and he first returned to the Parc des Princes a year ago in charge of Bayern Munich only to be sacked by the German giants after a chastening 3-0 defeat.
This time his Napoli team were excellent for long spells, fresh from beating Liverpool in their last European outing.
“We played well, and I think we would have deserved the victory for our efforts, but that is what playing PSG is all about, they have players of such quality,” said Ancelotti, who coached Di Maria at Real Madrid.
Mertens had hit the bar for Napoli before they went ahead just before the half-hour mark. Jose Callejon’s superb pass found Insigne, who finished beautifully over the advancing Alphonse Areola.
Tuchel knew action was needed and he switched to a back three for the second half, taking off struggling left-back Juan Bernat and replacing him with another centre-back in Thilo Kehrer.
Paris duly stepped up the pressure and were rewarded with the equaliser in the 61st minute, although there was a touch of good fortune about the goal.
Mbappe exchanged passes with Neymar before releasing Thomas Meunier, whose low ball in from the right was turned into his own net by Mario Rui.
Napoli retook the lead when Mertens stabbed home from close range in the 77th minute after Marquinhos had blocked a Fabian Ruiz shot, and that appeared to be that.
But just as PSG looked defeated, substitute Julian Draxler found Di Maria in the 93rd minute and he curled a shot high into the top-left corner from 20 yards.
“We have another opportunity at home and I hope to see the same game,” said Ancelotti.
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