Sergio Perez admits that he was "caught out" during the end of the Virtual Safety Car period as George Russell overtook him for third place.
Russell had been following Perez for much of the race, but saw an opportunity to pass following the VSC being called for Zhou Guanyu's stricken Alfa Romeo.
When the race returned to green flag conditions, Russell caught Perez unawares and overtook by getting up to speed quicker than the Red Bull driver.
Despite trying to fight back in the remaining laps of the race, Perez had to settle for fourth, and was left disappointed by the result.
Perez explains why Russell was able to overtake
Perez admits that the timing of the Virtual Safety Car period ending was what caught him out.
"It was a difficult afternoon," Perez told Sky Sports F1.
"First of all, we should have another podium. I don't know what was going on with the Virtual Safety Car. I just got caught up with it, because it was ending already from the exit of [Turn] 8, and it ended up at the end of [Turn] 15.
"So there was something going on there and I got caught up with it."
The Mexican has identified this as enabling Russell to catch him and overtake.
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It was a double podium result for Mercedes, with Lewis Hamilton taking second place ahead of Russell in third.
Perez acknowledges that the pace of the Silver Arrows was impressive at the Circuit Paul Ricard.
"They were quite strong, especially towards the end of that Medium stint," the Red Bull driver said.
"And George [was strong] towards the end. I think once the Virtual Safety Car ended, I was able to go a bit quicker, and it was closer at the end with George."
All in all, Perez was left struggling to find a balance he was happy with in his RB18.
"Today [was] just not a good day," he conceded.
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