ALEXANDRIA, Virginia (Reuters) - News Corp ( NWSA ) explored switching away from Google's advertising tools in 2017 but estimated that doing so would lose the Wall Street Journal publisher at least $9 million in ad revenue, a former executive testified on Tuesday at Google's antitrust trial in Virginia.

Google frustrated publishers by introducing features that benefited itself more than them, said Stephanie Layser, who worked in advertising technology at News Corp ( NWSA ) from 2017 to 2022. Despite those concerns, almost no one in the publishing industry used anything else, because Google's publisher ad server is tied to Google's ad exchange, she said.

"I felt like they were holding us hostage," Layser said on the stand.

She testified on the second day of what is expected to be a multi-week trial in which the U.S. Department of Justice will seek to show that Google monopolized markets for publisher ad servers, advertiser ad networks and the ad exchanges that connect the two.

NewsCorp documents shown at trial estimated that in 2016, the publisher made $83.3 million from ads sold instantaneously through ad tech tools. More than half of those transactions went through Google's ad exchange, with $18.4 million from Google ads advertisers.

The publisher estimated that around half of that, or $9 million, was exclusive to Google and would be lost in any shift to another product.

By the time she left, around 70-80% of News Corp ( NWSA ) ad transactions flowed through Google's ad exchange, Layser said.

Google has said that the case is based on an outdated look at the industry, and that large publishers use an average of six different platforms to sell ads that there are more than 80 such services.

At trial, prosecutors are seeking to show Google used dominant positions in technology for publishers and advertisers to keep them from using other tools and undercut bids placed through competitors' products.

If U.S. District Judge Leonie Brinkema finds that Google broke the law, she would consider prosecutors' request to make Google at least sell off Google Ad Manager, a platform that includes the company's publisher ad server and its ad exchange.