Trailing until the 89th minute against Sporting lisbon, Real Madrid was reminded that becoming the first club to retain the trophy in the Champions league era won't be an easy task.
The 11-time champions needed a late winner from substitute Alvaro Morata to prevail 2-1 in their opening game in the competition on a Wednesday night also marked by Premier league champion leicester's maiden Champions league win.
Playing against his former team, Cristiano Ronaldo was on the score sheet at Santiago Bernabeu stadium as he leveled with one minute left from a free kick before Morata's winner in added time. But the Portugal star was unable to match Sergio Aguero, who netted a hat trick with Manchester City. Sergio Aguero kept up his explosive start to the season by scoring a hat trick for Manchester City in a 4-0 win over Borussia Moenchengladbach in their rescheduled match.
After a one-year absence in the tournament, Dortmund celebrated its return by putting six goals past legia Warsaw, while French clubs Monaco and lyon made both winning starts.
Thrashed by Tottenham 4-1 at White Hart lane last season in Europa league, Monaco took its revenge on Europe's biggest club stage with a 2-1 win at Wembley. The match was moved to England's national stadium because of construction work at Spurs' home ground and drew 85,011 fans, a club-record home attendance and a record for an English club.
Monaco playmaker Bernardo Silva opened the scoring after only 15 minutes before Thomas lemar doubled the lead near the half-hour mark.
Spurs reduced the deficit through Toby Alderweireld's header in the 45th minute but couldn't make the most of lots of possession in the second half.
Bayer leverkusen failed to hold onto a 2-0 lead in the group's other game, letting CSKA Moscow come from behind and rescue a point in a 2-2 draw.
Elsewhere, Zinedine Zidane's coaching skills made the difference, as his decision to bring in Morata and James Rodriguez paid off. Rodriguez provided the assist, curling a perfect ball for Morata's header.
Sporting had the match at the Santiago Bernabeu Stadium largely under control and was rewarded when Brazilian midfielder Bruno Cesar gave scored with a shot from inside the area in the 48th minute. Ronaldo equalized from an 89th minute free kick.
Borussia Dortmund enjoyed a stroll in park at Polish champion legia Warsaw, scoring with six different players in a 6-0 rout.
Riyad Mahrez scored twice as leicester opened its maiden Champions league campaign with a comfortable 3-0 victory at Belgian champion Club Brugge.
Sixteen years after its last European game, the English Premier league champion was unfazed by the occasion and got off to a strong start, with Marc Albrighton scoring after just five minutes.
Mahrez then curled a free kick home and scored his second goal from the penalty spot. The victory gave leicester the lead in its group after FC Porto and Copenhagen drew 1-1.
lyon top Group H with 3-0 win over Dinamo Zagreb after Juventus was held to a disappointing 0-0 draw by Europa league winner Sevilla.
Playing with a depleted side, lyon relied on its homegrown players and the class of teenager Maxwel Cornet to open its Champions league campaign in style with a 3-0 defeat of Dinamo Zagreb.
lyon quickly opened the scoring courtesy of Corentin Tolisso's goal.
Jordan Ferri, who honed his skills at lyon's renowned academy with Tolisso, made it 2-0 four minutes into the second half from Cornet's assist. The 19-year-old Cornet then scored the third.