Key points
Rampant Hearts surged eight points clear of Celtic at the top of the Scottish Premiership to strengthen their early title credentials and worsen the malaise around Brendan Rodgers' side.
A superb goal from Kyziridis and a spot-kick from Shankland gave Derek McInnes' side a two-goal advantage that the title holders could not overcome.
The young defender - one of three youngsters in the Celtic starting lineup - scored an own goal early on to put the home side ahead at a energetic Tynecastle, and it was the youngster's infringement that led to the penalty.
A stylish finish from McGregor of the season had swiftly equalized the first goal, and Benjamin Nygren 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 regained dominance to hand the visitors to back-to-back losses for the first time since 2023.
It also gives the Hearts a commanding advantage at the summit.
The last time they were so far clear was in the 2005-2006 season under George Burley, when they ended as runners-up - one of just four occasions that a team has not secured the title having held such a lead in the last three decades.
McInnes, whose Aberdeen side were second to Celtic on multiple times, has been working hard to downplay championship discussions, as the club seek their maiden league title since 1960.
Rodgers has seen it all before at the club - both flawless and stuttering starts to a season. He's yet to finish second best.
But on the weekend's performance, this Hearts team showed they need to be taken seriously.
Hearts started with the self-assurance of a side unbeaten in the league this season - fans' favourite Kyziridis poked a cross in for Shankland to stab wide.
The Glasgow team didn't help themselves with some unusual sloppiness in possession and they faltered under their first real examination.
A headed effort from Braga was touched on to a post by the Celtic keeper and Shankland's return ball was turned into the net by the hapless defender, in for the injured Cameron Carter-Vickers.
Celtic found their rhythm and started building moves. From one such move, the midfielder released Tierney to square for the captain, who slotted home.
Aside from the Nygren chance, Celtic struggled to create much for striker Kenny or his substitute Yamada amid the unavailability of injured forwards their key strikers.
Hearts regained their intensity and adaptability in attacking from across the final third. The Greek forward, one of the finds of the club's relationship with shareholder Tony Bloom's Jamestown Analytics, fired a low shot past the goalkeeper from the edge of the box.
Immediately after, the defender tripped the midfielder in the right side of the box and Shankland scored. And it could've been an even more handsome win, had substitute Ageu not been stopped by the shot-stopper.
Celtic's second half was somewhat summed up in two incidents involving big-money recruit Engles.
The midfielder's wayward free-kick delivery was in stark contrast to his European display in the continental victory over Sturm Graz and the national team player was then yellow-carded for a cynical foul.
Hearts head coach Derek McInnes: "You see how hard the players work, the team spirit. That's not sufficient on its own to win games week in, week out.
"What we have got, is skillful players. You're seeing that from a lot of players who have been here previously. Some of the new signings are eager to show that.
"We're in a positive moment at the minute, but it's only three points. It's crucial 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 play it, they should have players offside but didn't. We have got to do better with the attempt.
"Plenty of matches to play for. It's still so early. It's only nine games in. Clearly, being eight adrift is not ideal. The positive is there's numerous fixtures to play.
"We now have to focus on the present and look to find more consistency and more quality in our play."
Celtic are back at home on midweek against promoted side Falkirk (19:45 GMT). At the identical hour in the top division, the Edinburgh outfit are at St Mirren.
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