When it comes to threats to advertising, Martin Sorrell is a keen student. He quickly identified Google as a frenemy during its rise. And now, it’s Amazon that’s keeping him up at night.

The reason: Amazon boasts a gigantic pool of data, not just likes and habits, but actual purchases. It could position its ad platform to be the arbiter of what ads work in actually driving people to make purchases — and the one best positioned to target those ads.

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