After weathering a frustrating first half in which Roma failed to finish off a series of promising attacks, Ronaldo broke the deadlock on 57 minutes when he ran on to Marcelo’s chip to beat Wojciech Szczesny with a shot that deflected off Alessandro Florenzi.

It was the Portuguese forward’s 12th goal in seven Champions League games this term.

To their credit, Roma dug deep in a bid to claim a share of the spoils and were unlucky not to earn a penalty when Florenzi went down under the studs-up challenge of Dani Carvajal.

Even after that decision, the hosts battled throughout a dominant late spell inspired by Egyptian winger Mohamed Salah, only to come up short.

It was inspiring stuff, but dozens of the 55,612 home crowd began leaving their seats five minutes from the finish when Jese flashed a shot across several defenders to beat Szczesny low at his far post.

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The victory gives Real a huge advantage ahead of the return leg at their Santiago Bernabeu home in three weeks time.

But Zinedine Zidane, who marked his full coaching debut in the competition by rejoicing with his players on the touchlines, called for caution.

“We’re all happy because winning here wasn’t easy but we we’re not through yet. We still have the second leg to play,” said the Frenchman, who was assistant to Carlo Ancelotti when Real won their historic 10th trophy in 2014.

“I’m happy for the way the lads played, especially when we improved in the second half. Everyone expects Ronaldo to score, and he fulfilled expectations.”

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Roma coach Luciano Spalletti was left lamenting Roma’s failure to finish off their impressive work in the final third of the pitch.

“We had so many chances tonight and I’m so disappointed we didn’t stick the ball in the net,” said Spalletti, who was in charge in 2008 when Roma ousted Madrid over two legs.

“The lads gave it everything, they played well but we just couldn’t score. And when you give Ronaldo an inch, he takes a mile.”

Diego Perotti, signed in January from Genoa, was the driving force throughout for the hosts, stealing possession, leading attacks and setting up Salah and El Shaarawy.

Real were restricted to a series of half-chances before the interval, the best of which came from Marcelo when he volleyed Ronaldo’s clever lob just wide of Szczesny’s far post.

Stephan El Shaarawy did well to run on to a long ball but came off worse when goalkeeper Keylor Navas rushed out to clear.

Ronaldo stunned the stadium into silence just before the hour when he ran onto Marcelo’s lob before turning inside and unleashing a shot that came off Florenzi to leave Szczesny clutching at thin air.

Dzeko replaced El Shaarawy just after the hour and soon after fans were up out their seats when Radja Nainggolan set up Salah, but the Egyptian could only toe-poke a weak shot at Navas while under pressure.

The hosts kept pressing but were lucky to see a Ronaldo’s header from a long cross on the right crawl past Szczesny’s far post.

Fans were up out their seats when Florenzi came down under Carvajal’s challenge, but match referee Pavel Kralovec was not convinced.

“I didn’t see it,” Zidane said.

Spalletti, meanwhile, refused to fan the flames.

“I don’t want to enter that debate. I do my job on the touchlines and it’s up to others to do theirs.”

Roma were finally subdued four minutes from time when Jese used several players as a screen before sending an angled drive past Szczesny.