Watched by millions around the world, the hotly-anticipated clash at the Bernabeu was Portuguese Ronaldo's first game against the club where he made his name before joining Real for a world-record fee in 2009.
It also renewed the rivalry between Real coach Jose Mourinho and his United counterpart Sir Alex Ferguson as the La Liga champions seek the 10th European title that has eluded them since 2002 and the Premier League leaders chase a fourth triumph in Europe's elite club competition.
"It's true they had two big chances in the second half but it was out of context because we were chasing the game and they were defending and waiting," the former Chelsea and Inter Milan manager told a news conference.
"Basically ,what I thought, everything's open, very open for the second leg. I think this will go to the last minute."
Ronaldo, top scorer in this season's Champions League with seven goals in seven appearances, barely celebrated his goal and spoke of the debt he owes United, where he was transformed from an inexperienced and fragile teenager into one of the world's most formidable forwards.
"It's a place where I played for six years and I have a lot of affection for the club as everyone knows," Ronaldo told Spanish television broadcaster Canal Plus.
"United are very strong in defence and they created some problems for us. But with the chances we had, we could have scored more goals," the 28-year-old added.
"Everything's open for the game at Old Trafford and I think we have the ability to score goals there and get through."
The two clubs were paired in Europe for the first time in almost a decade and as well as 2008 world player of the year Ronaldo the match showcased some of the game's top performers like United forwards Robin van Persie and Wayne Rooney and Real playmaker Mesut Ozil.
Real attacked the visitors' goal from the start, United goalkeeper David de Gea tipping a Fabio Coentrao effort on to a post in the sixth minute as a ragged United struggled to retain possession.
The Real faithful are hungry for Champions League success as the team's bid for a second straight La Liga title has floundered and they are stuck down in third 16 points behind leaders and arch rivals Barcelona.
De Gea, who joined United from Real's city rivals Atletico Madrid, again denied Coentrao on t
