WASHINGTON (Reuters) - The U.S. Justice Department's once-in-a-generation antitrust fight with Alphabet's Google (GOOGL.O) gets some star power on Monday when Microsoft (MSFT.O) chief executive Satya Nadella takes the witness stand.

The government is likely to ask Nadella about the obstacles posed by Google's dominance as Microsoft sought to grow Edge and Bing, its browser and search engine.

The government has argued that Google, worth more than $1 trillion with some 90% of the search market, illegally paid $10 billion annually to smartphone makers like Apple (AAPL.O) and wireless carriers like AT&T (T.N) and others to be the default search engine on their devices.

The clout in search makes Google a heavy hitter in the lucrative advertising market, boosting its profits.

Google has sought to show that the quality of its products is the reason for its success rather than illegal behavior.

Nadella became CEO of Microsoft in 2014, long after the tech giant had faced its own federal antitrust lawsuit. That court fight, which began in 1998 and ended in a 2001 settlement, forced Microsoft to end some business practices and opened the door to companies like Google.

As Google, which was founded in 1998, became an industry-leading search engine, the two became bitter rivals. Both have browsers, search engines, email services, and a host of other overlaps. They have recently become rivals in artificial intelligence, with Microsoft investing heavily in OpenAI and Google building the Bard AI chatbot among other investments.