Satya Nadella, Microsoft’s chief executive, testified on Monday that Google’s power in online search was so ubiquitous that even his company found it difficult to compete on the internet, becoming the government’s highest-profile witness in its landmark antitrust trial against the search giant.

In more than three hours of testimony in federal court in Washington, Mr. Nadella was often direct and sometimes combative as he laid out how Microsoft could not overcome Google’s use of multibillion-dollar deals to be the default search engine on smartphones and web browsers.

The internet was really the “Google web,” Mr. Nadella told the packed courtroom, adding that Google could now use its advantage and scale to build tools to dominate the emerging artificial intelligence industry.

The image of the chief executive of a leading tech rival — Microsoft is one of the world’s biggest public companies, valued at $2.4 trillion — saying it could not easily fight Google was striking. Mr. Nadella’s testimony underscored how entrenched Google has become in online search as the government seeks to prove that the company broke monopoly laws by striking anticompetitive deals to crush rivals.