The Bianconeri had lost 2-0 in Turin in the reverse fixture as an absent Ronaldo recovered from coronavirus, meaning they would have to improbably better that result at Camp Nou to leapfrog their hosts.
Cristiano Ronaldo scores his first goal against Barcelona in the Champions League 😳 pic.twitter.com/2DFHbWQzgR— Champions League on CBS Sports (@UCLonCBSSports) December 8, 2020
But Barca's recent defensive woes continued on Tuesday and two Ronaldo penalties - one contentious, the other routine - and a Weston McKennie volley carried Juve to a deserved victory.
WESTON MCKENNIE!— Champions League on CBS Sports (@UCLonCBSSports) December 8, 2020
The goal, the celebration 😍 pic.twitter.com/ycOTpt8Ym0
Ronaldo had not previously netted in five European matches against the Blaugrana yet delighted in piling further pressure on under-fire home coach Ronald Koeman, whose side are ninth in LaLiga.
The Portuguese - meeting Lionel Messi for the first time as a Juve player - stuck to the script as he made the most of a generous early penalty award after Ronald Araujo appeared to get his body between man and ball in the box, dispatching his spot-kick down the centre of the goal.
It was two after just 20 minutes - the earliest Barca have ever conceded twice in a home Champions League game - with the defence this time afforded no excuse.
McKennie moved the ball wide to Juan Cuadrado and was allowed to meet the subsequent cross completely unimpeded with a close-range scissor kick.
Half-time then disrupted an improving home display and Barca were dealt a further blow five minutes after the restart as Clement Lenglet was deemed to have handled, following a VAR check, and Ronaldo was again clinical from 12 yards.
Messi's free-kick was nodded onto the top of the crossbar by Antoine Griezmann, who was denied a penalty due to offside as pressure again built on the Juve goal and Matthijs de Ligt dived in rashly.
A VAR call belatedly went Barca's way when Ronaldo bundled the ball onto an offside Leonardo Bonucci, seeing a fourth goal struck off, but it did not impact an eye-catching result.