Bale made it seven goals in his last nine Real games - the second against Celta was a stunning individual effort - to boost his hopes of starting for the European champions against Liverpool in Kiev on May 26.
Bale burst onto Luka Modric's pass to smash home a low opener and scored a fantastic second, cutting in from the right, nutmegging Jonny Castro and shooting from a hard angle.
Two minutes later it was 3-0 when creator turned goal-scorer as it was Isco's turn to curl the ball into the top corner with his right foot.
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Zidane's team continued to knock on the door down the other end though, and doubled their advantage in the 30th minute when Bale cut inside before finding the top corner with a wonderful finish.
Isco added the third goal just moments later with a strike from 20 yards.
Gareth Bale bagged a brace while Isco, Achraf Hakimi and Toni Kroos were also on target for Los Blancos as they put Celta to the sword in the Spanish capital, where Sergio Gomez scored at the wrong end to make it six on the night.
The Celta goalkeeper kept out a header before the hour to deny Bale his hat-trick and the officials erred in disallowing Benzema's follow-up for offside. Bale nearly got his hat-trick but his effort was saved by the Celta Vigo goalkeeper, Zidane substituted Karim Benzema for Borja Mayoral in the 64th minute and Isco was removed for Marco Asensio and in the 72nd minute Bale was substituted for Lucas Vazquez.
The home side took an early lead when Bale beat the offside trap to run onto a Luka Modric through ball for a finish in the bottom corner on 13 minutes.
Real thumped Celta 6-0 on the night.