Bale scored one and then provided the cross that led to another as Lucas Vazquez wrapped up a clinical 2-0 win at the Stadio Olimpico.
Roma squandered several chances in the first half, the best of them falling to Cengiz Under, who was guilty of a shocking miss from six yards, with the goal gaping.
Federico Fazio’s mistake then gifted Bale the opener and Roma faded thereafter, allowing Madrid to erase memories of their shock defeat to Eibar last weekend by finishing top of Group G.
“It was important to bounce back quickly,” Real coach Santiago Solari said. “We are very happy.”
Both teams knew they had already qualified thanks to Viktoria Plzen’s win over CSKA Moscow in the early kick-off, making this a golden chance to secure first place.
Madrid seized it, showing once again their immunity in the Champions League, where the team’s problems in domestic competitions are seemingly never transferred.
They have gone more than eight hours without a goal and two weeks without a permanent coach but Santiago Solari still leads Real into the knock-out stages as group winners with one game to spare.
“We like playing against the best – it brings out the best in us,” Bale said.
Solari might have felt relief after the disastrous display at Eibar on Saturday while the pressure mounts for Roma coach Eusebio di Francesco. They have won only one of their last five Serie A games, and Antonio Conte was watching on in the crowd.
“We’re playing against a side of champions,” di Francesco said. “The only difference was they scored with their first chance.”
There have been a fair few winners from Solari’s time at the helm but Isco is not one of them. After not starting any of the new coach’s first five games, he failed even to make the bench in Italy.
“It was a sporting decision, nothing else,” Solari said. “It is up to all players to be at 100 per cent for the coaches to pick them.”
A breathless first half was dominated by Roma but Madrid had the first chance. Vazquez plucked Bale’s cross out of the sky and cut back for Luka Modric, whose deflected shot was well saved by the foot of Robin Olsen.
From there, the best chances belonged to the hosts. Raphael Varane diverted away from the boot of Patrik Schick and over his own bar. Stephan El Shaarawy hobbled off and his replacement, Justin Kluivert shot over. Schick failed to connect from close range.
Madrid were hanging on, with one Roma assault leaving a handful of white shirts sprawled across the penalty area. Thibaut Courtois saved with his feet, Dani Carvajal blocked, before Aleksandar Kolorov fired off a trademark shot from range that whistled wide.
The worst miss of the half, and perhaps the season, was to come. Carvajal dallied in the corner, teeing up Nicolo Zaniolo to cross. At the back post, Madrid had not recovered, neither had Courtois, who was scrambling from the other side of the goal. Under, without pressure, blazed over.
He trudged off at half-time and was consoled like he had missed a penalty, but it seemed inevitable Roma would pay for the error.
They did, twice in 12 minutes. First, Fazio compounded Olsen’s poor clearance with a worse header back, which landed at the feet of Bale and he made no mistake.
Then Bale was involved in the second, a clipped cross finding Karim Benzema, whose clever header down gave Vazquez a simple finish.
In between, Bale could have made it two himself when he was set free on the break, as Roma lost momentum, shape and belief.
Bale, Benzema and Modric were all taken off, rested for Valencia on Saturday, as an initially close contest became comfortable.
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