'We played an exceptional second half, but the first one we forgot to be there,' Guardiola told BT Sport. In photo: Monaco players celebrate at the end of thier match against Manchester City. Photograph: (AFP)
The two sides were 6-6 on aggregate after both the home and away match but Pep's side had to go on the away goals rule
Tiemoue Bakayoko's superb header fired Monaco into the Champions League quarter-finals as Wednesday's 3-1 victory condemned Pep Guardiola's Manchester City to a painful exit on away goals.
Trailing 5-3 from the first leg in England, teenage starlet Kylian Mbappe put Monaco ahead on the night after just eight minutes at the Stade Louis II and Brazilian midfielder Fabinho converted the home side's second just before the half hour.
Leroy Sane's 71st-minute goal handed City a 6-5 advantage on aggregate, but Bakayoko turned home Thomas Lemar's free-kick 13 minutes from time as Monaco claimed a famous win.
Monaco go through to Friday's draw for the last eight in Nyon carrying French hopes following Paris Saint-Germain's shock elimination, while Guardiola suffered his earliest Champions League exit as a manager.
"We played an exceptional second half, but the first one we forgot to be there," Guardiola told BT Sport.
"In the second half we were much, much better but it was not enough. It happens and we'll learn, we have a team with not a lot of experience in this competition."
Monaco hero Bakayoko will miss the first leg of the quarter-final through suspension but was delighted to advance.
"It's an immense joy to score in the Champions League. I didn't expect this," he said.
"Over two games we deserved to qualify. Now we want to keep going."
The French league leaders suffered a blow before kick-off as Radamel Falcao, who scored twice in the first leg, failed to recover from a hip injury in time and was replaced by the versatile Valere Germain.
Jemerson returned in place of the suspended Kamil Glik in defence, while Guardiola, who took charge of his 100th game in Europe, made two changes with Gael Clichy and Aleksandar Kolarov given the nod ahead of Nicolas Otamendi and Yaya Toure.
Coach Leonardo Jardim had challenged Monaco to draw inspiration from the side which dumped out Real Madrid en route to the 2004 final, recovering from a 4-2 defeat in the first leg of that year's quarter-finals before knocking the Spanish giants out on away goals.
And the hosts were inspired by 18-year-old Mbappe, who has emerged as one of Europe's most promising talents amid Monaco's thrilling pursuit of the French title this season.
Fast start dazzles City
Willy Caballero stood tall to repel Mbappe's crisp early strike as he knifed through the City defence, but the Argentine goalkeeper was helpless as the teenager poked home Bernardo Silva's driven low cross just a minute later.
City recovered from twice falling behind in the first leg, but the opening goal emboldened Monaco as Germain unleashed a searing strike from distance that zipped not too far over.
Guardiola had emphasised the best form of defence would be attack for City, yet the visitors struggled to settle and Monaco grabbed their second goal in not dissimilar fashion on 29 minutes.
Lemar fed the ball into the overlapping run of Benjamin Mendy, a reported target of City's this summer, and the left-back picked out Fabinho to side-foot beyond Caballero.
Mbappe's electrifying pace posed a constant threat for City, but the Premier League side gradually began to pose Monaco problems as well although Raheem Sterling's hesitancy allowed the hosts to clear as the England winger looked to tee up Sergio Aguero.
The Argentine striker then skied over from close range after Sane skipped past Andrea Raggi down the the left-hand flank, before Danijel Subasic thrust himself superbly in the way after Aguero was picked out by David Silva inside the area.
Sane broke in behind the Monaco backline once more but steered his effort the wrong side of the near post with Subasic charging off his line, but City finally broke through with 19 minutes left.
Sterling broke in from the right and his low shot was palmed away by Subasic, but Sane reacted quickest to thump the rebound into the roof of the net and change the complexion of the tie.
However that merely drew an immediate response from Monaco as Lemar whipped in a devilish free-kick that Bakayoko expertly planted into the far corner to consign Guardiola to a first-ever last 16 European exit.