At a glance
Rampant Hearts powered eight points clear of their Glasgow rivals at the top of the Scottish Premiership to bolster their early championship claims and deepen the malaise around Brendan Rodgers' side.
A superb goal from Kyziridis and Lawrence Shankland's penalty gave Derek McInnes' side a 3-1 lead that the title holders could not recover from.
The young defender - one of three youngsters in the Celtic XI - scored an own goal in the early stages to put the home side ahead at a energetic their home ground, 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, choosing placement over power and allowing Alexander Schwolow to make a crucial stop.
But the unstoppable Hearts side seized control again to consign Celtic to back-to-back losses for the initial occasion since 2023.
It also gives the Edinburgh side a commanding advantage at the summit.
The last time they were this far ahead was in 2005-06 under their former manager, when they ultimately finished second - one of just four occasions that a team has failed to win the title after building such an advantage in the last three decades.
McInnes, whose former team were second to Celtic on four occasions, has been at pains to play down title talk, as the club seek their first top-flight crown since 1960.
The Celtic boss has experienced this previously at the club - both perfect and shaky beginnings to a season. He's yet to finish second best.
But on Sunday's evidence, this Edinburgh outfit showed they deserve serious consideration.
Hearts started with the confidence of a side unbeaten in the league this season - fans' favourite Kyziridis delivered a ball for Shankland to fire off target.
Celtic didn't help themselves with some uncharacteristic carelessness in possession and they buckled under their initial test.
Claudio Braga's header was tipped onto the woodwork by the Celtic keeper and the follow-up from Shankland was turned into the net by the unfortunate Murray, in for the absent center-back.
Celtic grew into the contest and started stringing passes together. From one such move, the midfielder released the full-back to square for McGregor, who rolled in.
Aside from the opportunity for Nygren, Celtic struggled to create much for lone frontman Jonny Kenny or his replacement Shin Yamada amid the unavailability of stricken attackers Kelechi Iheanacho and Daizen Maeda.
The home team rediscovered their energy and versatility in attacking from across the final third. Kyziridis, a standout performer of the club's scouting network, fired a low shot past Schmeichel from the edge of the box.
Immediately after, the defender fouled the midfielder in the inside right channel and Shankland scored. And it could've been an more comprehensive victory, had substitute Ageu not been denied by the shot-stopper.
The visitors' performance after the break was partially encapsulated 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 Europa League win over their Austrian opponents and the national team player was then booked for a professional foul.
The Hearts manager: "The effort from the squad is evident, 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 before. Some of the new signings are keen to demonstrate their quality.
"We're in a positive moment at the minute, but it's only three points. It's crucial for us to maintain this form. We've got much work ahead, loads to do. After the first two rounds is typically a benchmark of where you are."
Celtic manager Brendan Rodgers: "The second goal was disappointing from our perspective. We didn't move when the ball went back as a line so when they play it, they should have players offside but weren't. We have got to improve with the attempt.
"Plenty of matches to play for. It's only the beginning. It's just nine matches. Clearly, being eight points behind 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 better performance in our play."
The Glasgow club are back at home on midweek against promoted side Falkirk (evening kick-off). At the same time in the top division, the Edinburgh outfit are at St Mirren.
A former casino floor manager turned slot analyst, Mikael shares data-driven insights to help players make smarter betting decisions.