Mr. Nadella’s appearance on the witness stand in the case — U.S. et al v. Google, which is the first monopoly trial of the modern internet era — was also a sign that the bitter rivalry between Microsoft and Google continues unfettered. Over more than two decades, the two companies have battled over online search, mobile computing, web browsing and cloud computing and dueled in multiple legal battles as both became ever more powerful. Now the companies are locked in an increasingly intense fight over A.I.

“Despite my enthusiasm that there is a new angle with A.I., I worry a lot that this vicious cycle that I’m trapped in could get even more vicious,” Mr. Nadella said.

Regulators around the world have been working to rein in the power and reach of Google, Apple, Amazon and Meta, which owns Facebook, Instagram and WhatsApp. Last week, the Federal Trade Commission sued Amazon, arguing it broke antitrust laws by squeezing merchants on its site. The F.T.C. has also filed an antitrust lawsuit against Meta, claiming it snuffed out nascent rivals, and the Justice Department has sued Google in a second case over its control of online advertising.

The 10-week Google trial is being closely watched as a referendum on whether the government can slow down Silicon Valley’s biggest companies. A Google victory could be a major rebuke of regulators who say the tech giants have too much sway over their customers, partners and start-up competitors.

At the heart of the government’s case against Google is the accusation that the company illegally cemented its monopoly in online search by paying to be the default search engine on browsers like Apple’s Safari and Mozilla’s Firefox, as well as on the home screen of smartphones. Google has argued that the default positions are not overwhelmingly powerful and that users can switch to a new search engine if they like.