The championship leader and four-time champion had to pull his car to a halt in the pit lane in the closing stages of third and final practice when Ferrari detected a problem.
Instead of working on repairing the water leak problem, the team decided to fit a new power unit immediately, the German's third of the year's permitted supply of four.
This meant they had only two hours to complete the work before the start of Saturday afternoon's qualifying session.
As the engine work began, Mercedes non-executive boss Niki Lauda dismissed rumours that Vettel was set to leave Ferrari for the German outfit.
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"I'm sorry, I'm very sorry, but it drives me crazy and I can tell you a reason you'll understand," he told Sky Sports.
"A Ferrari driver, his head is for Ferrari, and the money in the end is also there. Why would Vettel leave Ferrari when he's winning?
"There's no reason. He will stay in his team, he loves his team, everything is right. So, there's no reason to think these things.
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