Erling Haaland's double earned Manchester City a 2-0 win at FC Porto - but it was another game where Enzo Maresca's side won but struggled to convince.
City's main man up front was surprisingly wasteful in the first half, seeing two glorious chances in the area saved by Diogo Costa as well as heading another good chance over the bar.
Porto's goalkeeper was in inspired form in the opening 45 minutes, also saving well from Rayan Cherki from the rebound of one of Haaland's chances.
But Haaland took just two minutes of the second period to make amends, barging Jakub Kiwior aside to nod an Enzo Fernandez cross in off the post.
But City then took their foot off the gas as the Portuguese champions started to grow in confidence. Porto failed to produce a shot on target but Alan Varela curled just wide of the far post from the edge of the box in a warning shot.
Then Gianluigi Donnarumma was nearly caught in possession by Porto forward William Gomes, with the City goalkeeper lucky to see the ball run away to safety. City did find a way to seal the result, however, as Haaland tucked home Rayan Ait-Nouri's cutback in stoppage time.
Despite another game this season where City have won but struggled to impress at the same time, Maresca failed to win away of his three away Champions League ties as Chelsea boss last term.
He will see a winning start to this campaign as a positive - especially as City have become the first team to beat Porto at the Estadio do Dragao this calendar year.
But improvement is needed ahead of Sunday's Manchester derby at Old Trafford, live on Your Site.
16: SAVE! Antoine Semenyo starts the game brightly and curls an early effort into Diogo Costa's gloves.
21: BIG CHANCE FOR HAALAND! Semenyo feeds Phil Foden who clips a cross into the box for Haaland - who is denied by a smart glove from Costa.
27: BIG MISS! Haaland heads over another good chance via a cross from the left, this time from Semenyo.
29: BIG SAVE! Foden goes on a mazy run in and around the penalty area and forces another save from Costa.
31: DOUBLE SAVE! A man possessed! Costa denies Haaland's low drive from inside the box, then gets up to stop Rayan Cherki at the rebound.
42: OUCH! Marc Guehi catches Gabri Veiga with his arm which causes a lot of blood. With Porto calling for a red card for a violent elbow, the Man City defender avoids punishment. It was more accidental rather than malicious.
47: GOAL! Haaland takes just two minutes to rectify his wasteful first half by nodding a header in off the post from Enzo Fernandez's cross.
49: NO PENALTY! City goalkeeper Donnarumma clatters into Porto winger Pepe in the box… but the offside flag goes up against the forward.
57: SO CLOSE! Porto midfielder Alan Varela gets a chance to shoot and curls just wide of the far post. That was Porto's first shot of the game.
66: NERVY! Donnarumma is nearly caught in possession by Porto forward Gomes, but the ball runs behind for a goal kick.
85: JUST WIDE! City substitute Ait-Nouri has the chance to seal the result but steers a shot wide of goal from a good area.
90+1: GOAL! Haaland seals the result by slotting home Ait-Nouri's cutback after he did well to get to the byline.
Man City striker Erling Haaland to Amazon Prime:
"I hope so, we win away in a difficult game. Look at this atmosphere, it was unreal at times. So I hope [yes].
"Amazing game against the champions of Portugal. We knew it was going to be a difficult fight, man against man. They're a top team, impressive, all of them. They play amazing football.
"Well done to them. But with all the pressure, we have to deliver on the biggest stage. That's what we did tonight.
"Honestly we controlled the whole first half. Of course, it's a bit uncomfortable when they come high. But we created a lot of chances. I could have easily had a few more goals.
"But second half we scored and they managed to push us for a good 25 minutes. We managed to hold through and finish the game.
"These are the games we have to win. We had a great start to the season and in the Champions League, you have to win games. If not, you get all the pressure. You never know what's going to happen.
"We have to learn. Enzo has to demand a lot from us and each other. At Manchester City, you have to win games and win trophies, that's what we have to do."
Man City head coach Enzo Maresca to Amazon Prime:
"We could have scored more goals, especially in the first half. But overall we deserved to win the game. We created enough chances to win the game, we didn't concede a lot.
"It's normal to come here and concede some noises so overall I'm happy with the performance.
"If you think you're going to come here in this stadium and just play and not concede anything, it's not reality. It's normal that they are going to create something. They didn't create a lot - some actions, some mistakes from us. But I think it's normal."
Enzo Maresca has a point. You can't turn your nose up at winning away at FC Porto - an intimidating atmosphere, one that is not used to suffering defeat. Doing so in a manner where they conceded little in terms of clear chances is also a positive.
But look at City's performances this season. Trounced in the Community Shield by Arsenal, then a late show to overcome Bournemouth with a fortunate win. Then came a resounding win over Crystal Palace, but then they nearly dropped the ball in their 1-0 win over Coventry.
This was another game where they could have dropped the ball. City dominated for 47 minutes but then allowed FC Porto back into it. There were slack passes and constant losses of possession - a better team would have found a way back into the game.
And City face a better test on Sunday. No team will be more desperate to take advantage of such play than Manchester United at Old Trafford. It will be another big atmosphere for Maresca's City to overcome. But if they continue to play with fire, they will get burnt.
