WASHINGTON, May 3 (Reuters) -

Google and the U.S. Justice Department clashed in court on

Friday over claims that the Alphabet unit unlawfully

schemed to dominate search advertising, during closing arguments

in a case the government contends could shape the "future of the

internet."

U.S. District Judge Amit Mehta in Washington grilled

both sides with questions, probing whether competitive platforms

such as ByteDance's TikTok and Meta's Facebook and Instagram are

competitive substitutes for search advertising dollars.

Mehta called platform "substitutability" for advertisers

a central issue the court must resolve, as he prepares to render

a major decision in coming months on whether Google's conduct

broke antitrust law.

The judge also questioned whether Google assesses

competitors' pricing before making its own adjustments. Google's

advertising business is responsible for about three quarters of

its revenue.

U.S. government lawyer David Dahlquist argued that

"advertising revenue is what drives Google's monopoly power

today."

Google boasted it has no real market pressure, Dahlquist

said, arguing that the company does not fear increasing its

pricing or not improving its products. "Only a monopolist can

make a product worse and still make more money," Dahlquist

argued.

Google's lawyer John Schmidtlein on Friday countered

that Google's share of U.S. digital advertising revenue has

steadily decreased. He touted the advertising power of rival

platforms ByteDance's TikTok, Meta's Facebook and Instagram, and

Amazon ( AMZN ).

Schmidtlein argued that Google is "constrained" by rival

platforms "where the eyeballs are," because advertisers know

there are overlapping audiences and can spend dollars away from

Google.

He also asserted that Google was continually moving to

innovate its search advertising products. "If Google is a

monopolist, why improve anything? Why not just jack the price

up?" he told the court.

The Justice Department has hammered away at Google in a

trial that started on Sept. 12, contending the search engine

giant is a monopolist that illegally abused its power to boost

profits.

Witnesses from Verizon, Android maker Samsung

Electronics ( SSNLF ) and Google itself testified about the

company's annual payments - $26.3 billion in 2021 - to ensure

that its search is the default on smartphones and browsers, and

to keep its dominant market share.

Mehta is expected on Friday to take up the government's

claim that Google intentionally destroyed internal documents

that were relevant to the issues in the lawsuit. The government

wants Mehta to presume that Google deleted chats that were

unfavorable to the company.

Google has defended its data preservation practices, calling

them reasonable, and urged the court not to sanction the

company.

The court is not expected to issue an oral ruling at the

conclusion of the argument.

This case, filed by former President Donald Trump's

administration, was the first of five aimed at reining in the

market power of tech leaders.

The second, against Facebook parent Meta, was also filed

during the Trump administration. President Joe Biden's antitrust

enforcers have followed with a second case against Google and

cases against Amazon.com ( AMZN ) and Apple Inc. ( AAPL )