Google has long dominated the market for U.S. search ads, those marketing messages that frequently pop up at the top and bottom of your search results. And small wonder; it did, after all, turn its corporate name into a synonym for online search.

But now competitors like Amazon are starting to eat into Google’s market share, at least in specific areas. Such as, in Amazon’s case, product searches.

The U.S. search ad market overall will grow nearly 18% this year to reach $55.17 billion, according to eMarketer.

Amazon and other smaller players are focusing on particular niches for growth. Since many product searches start directly on Amazon, advertising in those results is an increasingly attractive prospect for marketers.

With Amazon, advertisers “can reach consumers during product queries, a time when they’re ready to buy,” said eMarketer principal analyst Nicole Perrin.

It doesn’t hurt that Amazon has improved its measurement and targeting tools, she said.

EMarketer predicts Google’s search ad market share will decline to 70.5% by 2021, from 73.1% in 2019. Meanwhile, Amazon’s share will grow from 12.9% to 15.9% by 2021, according to the research firm.