Google will continue to dominate the market for serving up ads along with search results, but Amazon and "smaller players" including Walmart, Target, eBay, and Pinterest are seeing their shares grow, the market tracker said.
Ads displayed along with query results at shopping sites are seen as having good odds of catching the eyes of consumers while they are intent on buying something.
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"Amazon's ad business has attracted massive increases in spending because advertisers can reach consumers during product queries, a time when they're ready to buy," eMarketer principal analyst Nicole Perrin said.
"Amazon has also rolled out better measurement and targeting tools, making it even more attractive for advertisers."
The US search ad market overall will have grown nearly 18% this year to reach $55.17 billion, with Google taking in $40.33 billion, or slightly more than 73%, eMarketer forecast.
While Google will remain dominant in the market, its share will drop to 70.5% by 2021, according to the forecast.
Meanwhile, the Seattle-based e-commerce colossus will see its share of the search ad market grow from 12.9% this year to 15.9% by 2021.
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"Polling suggests that most product searches now begin on Amazon, causing the No. 2 search player to grow rapidly and steal share from its larger rival," the forecast said.
Amazon's search ad revenue was predicted to grow nearly 30% this year to $7.09 billion.
Last year, the company took the second-place spot in the US search ad market from Microsoft, which was forecast to have a 6.5 per cent share this year, eMarketer said.
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