At a glance
Rampant Hearts powered eight ahead of Celtic at the summit of the Scottish top flight to strengthen their early-season championship claims and deepen the struggles around Brendan Rodgers' side.
Alexandros Kyziridis' fine strike and a spot-kick from Shankland gave the Hearts team a two-goal advantage that the title holders could not overcome.
Dane Murray - among three youth players in the Celtic XI - scored an own goal early on to put the home side ahead at a energetic their home ground, and it was the youngster's infringement that led to the spot-kick.
A stylish finish from McGregor of the season had swiftly equalized the first goal, and the Swedish attacker almost put Rodgers' team in front, opting for finesse over power and allowing the Hearts keeper to make a crucial stop.
But Derek McInnes' irrepressible Hearts seized control again to hand Celtic to successive league defeats for the initial occasion since 2023.
It also gives the Edinburgh side a significant lead at the summit.
The last time they were so far clear was in the 2005-2006 season under their former manager, when they ultimately finished second - one of just four occasions that a team has not secured the title having held such a lead in the past 27 years.
McInnes, whose former team were second to Celtic on four occasions, has been at pains to downplay championship discussions, as Hearts seek their maiden league title since 1960.
The Celtic boss has experienced this previously at Celtic - both perfect and shaky beginnings to a season. He's yet to finish second best.
But on the weekend's performance, this Edinburgh outfit showed they need to be taken seriously.
Hearts started with the self-assurance of a side unbeaten in the league this season - the popular attacker delivered a ball for Shankland to fire off target.
The Glasgow team didn't help themselves with some uncharacteristic carelessness in possession and they buckled under their first real examination.
A headed effort from Braga was tipped onto the woodwork by the Celtic keeper and Shankland's return ball was diverted goalwards by the unfortunate Murray, in for the injured Cameron Carter-Vickers.
The away side found their rhythm and started stringing passes together. From a flowing attack, Sebastian Tounekti released the full-back to set up McGregor, who slotted home.
Apart from the opportunity for Nygren, Celtic struggled to create much for striker Kenny or his substitute Yamada amid the unavailability of injured forwards Kelechi Iheanacho and Daizen Maeda.
Hearts rediscovered their energy and adaptability in attacking from across the final third. Kyziridis, one of the finds of the club's relationship with shareholder Tony Bloom's Jamestown Analytics, struck low past the goalkeeper from the just outside the area.
Almost straight away, Murray fouled Braga in the right side of the box and the penalty taker converted. And it could've been an more comprehensive victory, had replacement Ageu not been stopped by the shot-stopper.
The visitors' performance after the break was somewhat summed up in two incidents involving record signing Arne Engles.
The set-piece from Engles delivery was in stark contrast to his midweek exploits in the Europa League win over Sturm Graz and the national team player was then yellow-carded for a professional foul.
Hearts head coach Derek McInnes: "The effort from the squad is evident, the togetherness. That's not sufficient on its own to win games week in, week out.
"Our strength lies in, is skillful players. You're witnessing it from a lot of players who have been here before. Recent additions are keen to demonstrate their quality.
"We're in a positive moment at the minute, but it's just one win. It's important for us to keep winning. We've got much work ahead, loads to do. After the first two rounds is typically a benchmark of your standing."
Celtic manager Brendan Rodgers: "It's a really poor goal from our perspective. We didn't move when the play was recycled as a defensive unit so when they pass the ball, they ought to be caught offside but weren't. We have got to do better with the attempt.
"Plenty of matches to play for. It's still so early. It's just nine matches. Clearly, being eight points behind is not ideal. The comfort is there's numerous fixtures to play.
"Our attention must turn on the present and look to find more consistency and better performance in our play."
The Glasgow club are returning to their stadium on midweek against promoted side Falkirk (evening kick-off). At the same time in the Scottish Premiership, Hearts are at their opponents' ground.
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