Monaco scored twice in the first half hour to beat Tottenham 2-1 at Wembley in their Champions League Group E opener.
In front of the largest home crowd ever (85,011) to watch an English club, Spurs failed to deal with Monaco's counter-attacking threat as they conceded two goals on the break, the first of which was a superb strike from Bernardo Silva.
Thomas Lemar fired in a second and the game looked to be slipping away from Spurs until Toby Alderweireld pulled one back with a powerful header just before half-time.
But Monaco withstood strong pressure from Spurs in the second half to see out the match with their lead intact and inflict a first defeat of the season on their hosts.
Tottenham manager Mauricio Pochettino will hope the result - and the tepid nature of his side's performance at times - is not a signal that they are going to struggle as they make the national stadium their home in the Champions League this season while work on a new stadium gets under way.
Spurs settled into the occasion quicker than their opponents and Son Heung-min would have opened the scoring after nine minutes had Andrea Raggi not blocked his shot after Harry Kane's excellent cross.
Kane looked Spurs' most effective outlet on the right flank and he caused Monaco a number of problems before the visitors scored a brilliant counter-attacking goal.
Fabinho won possession in midfield and launched the ball forward for the pacy Portuguese midfielder Bernardo, who darted into the box and applied a deadly finish that left Hugo Lloris no chance.
Spurs countered immediately and Erik Lamela forced a good save from Danijel Subasic, but Monaco seemed to grow in confidence and began to dictate the rhythm of the game.
With just over half an hour played, Monaco extended their lead when Lemar - on as a substitute for the injured Nabil Dirar - pounced on a knock down from Djibril Sidibe's cross and lashed the ball high into the net past Lloris.
As the half looked set to peter out, Alderweireld gave Spurs a lifeline, powering Erik Lamela's corner into the net from close range to give the Wembley crowd some hope.
Pochettino sent on Mousa Dembele for Son after half-time and Spurs went close to levelling the match when Dele Alli unleashed a dipping 25-yard drive that Subasic did well to tip over the crossbar.
Dembele's presence seemed to free Alli to play at his best and his deft flick put Kane in on goal, but Jemerson blocked the England striker's shot.
Substitute Vincent Janssen gave Spurs a fresh attacking outlet in the closing stages of the game and he teed Kane up for a gilt-edged chance after 79 minutes, but Subasic was equal to his shot.
As the clock ran down, Monaco frustrated the Wembley crowd by playing comfortable possession football that ensured a late charge for a Spurs equaliser never materialised.