Key points
Rampant Hearts powered eight points clear of Celtic at the top of the Scottish Premiership to strengthen their early-season title credentials and worsen the malaise around Brendan Rodgers' side.
A superb goal from Kyziridis and Lawrence Shankland's penalty gave Derek McInnes' side a two-goal advantage that the title holders could not recover from.
Dane Murray - one of three youngsters in the Celtic starting lineup - scored an own goal in the early stages to put the home side ahead at a bouncing Tynecastle, and it was the youngster's infringement that led to the spot-kick.
Callum McGregor's classy first goal of the season had swiftly equalized the first goal, and the Swedish attacker almost put Celtic in front, opting for finesse over power and allowing the Hearts keeper to make a crucial stop.
But the unstoppable Hearts side seized control again to consign Celtic to back-to-back losses for the first time since last year.
It also gives the Edinburgh side a significant lead at the summit.
The last time they were so far clear was in 2005-06 under George Burley, when they ended as runners-up - one of just four occasions that a team has failed to win the title having held such a lead in the last three decades.
McInnes, whose Aberdeen side were second to Celtic on four occasions, has been working hard to play down championship discussions, as the club seek their first top-flight crown since 1960.
The Celtic boss has seen it all before at Celtic - both flawless and stuttering starts to a season. He's yet to finish second best.
But on Sunday's evidence, this Edinburgh outfit showed they deserve serious consideration.
The Edinburgh side 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 aid their cause with some unusual sloppiness in possession and they faltered under their first real examination.
A headed effort from Braga was tipped onto the woodwork by Kasper Schmeichel and Shankland's return ball was diverted goalwards by the hapless defender, in for the injured Cameron Carter-Vickers.
Celtic found their rhythm and started stringing passes together. From a flowing attack, Sebastian Tounekti released the full-back to square for McGregor, who rolled in.
Aside from the opportunity for Nygren, the visitors found it hard to generate chances for lone frontman Jonny Kenny or his substitute Yamada amid the absence of stricken attackers Kelechi Iheanacho and Daizen Maeda.
The home team rediscovered their energy and adaptability in creating chances in the opposition half. Kyziridis, a standout performer of the club's relationship with shareholder Tony Bloom's Jamestown Analytics, struck low past the goalkeeper from the edge of the box.
Almost straight away, Murray tripped Braga in the inside right channel and Shankland converted. And it could've been an more comprehensive victory, had replacement Ageu not been stopped by the shot-stopper.
Celtic's second half was somewhat summed up in a couple of moments involving record signing Arne Engles.
The set-piece from Engles delivery was in sharp opposition to his European display in the continental victory 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 consistently.
"Our strength lies in, is good technical ability. You're witnessing it from multiple individuals who have been here previously. Recent additions are eager to show that.
"We're in a positive moment at the minute, but it's just one win. 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 where you are."
Celtic manager Brendan Rodgers: "It's a really poor goal from our perspective. We didn't move when the ball went back as a defensive unit so when they pass the ball, they should have players offside but didn't. We have got to improve with the shot.
"Plenty of matches to play for. It's still so early. It's just nine matches. Clearly, being eight points behind is not what we would want. The comfort is there's so many games to play.
"Our attention must turn on the here and now and look to find more consistency and better performance in our play."
The Glasgow club are back at home on Wednesday against promoted side Falkirk (19:45 GMT). At the same time in the Scottish Premiership, the Edinburgh outfit are at St Mirren.
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