In Silicon Valley today, venture capitalists routinely tell start-ups to steer clear of Google’s plans, as they once did with Microsoft. “I tell them to know what Google’s flight path is,” said Mitchell Kapor, a venture investor and former technology executive, who competed against Microsoft. “If you do something that falls under their shadow, you feel a chill.”

Still, Mr. Kapor said, the industry environment is quite different today. “The Google guys grew up with the Internet and its openness,” he said. “Without the open Internet, there would be no Google, and they understand that.”

Others point to another force that helped Google — government action. “Without the antitrust case and the restraints on its behavior, Microsoft would have built search into the operating system and we might not have had Google,” said Christine Varney, a lawyer who represented Netscape in Washington and is the former head of the antitrust division the Justice Department in the Obama administration.

The Google claim that the company faces genuine competition today in its key markets like search and search advertising is an exaggeration, industry executives say.

But things are more open, as Mr. Kapor says. And there are other digital giants in technology — Apple, Facebook and Amazon — in nearby markets with designs on parts of Google’s business, in a way that was not the case in Microsoft’s heyday. For example, 30 percent of online shoppers in America begin their search on Amazon, more than twice the share for Google.

“Google inhabits a larger, more complex and faster-moving environment than Microsoft did in the 1990s,” observed William E. Kovacic, former chairman of the F.T.C. “So the difficulty of both diagnosis and remedy seems greater than in the case of Microsoft.”

Mr. Reback and Ms. Creighton are both products of elite universities; he is Yale and Stanford Law, she is Harvard and Stanford Law. But former colleagues say the two have sharply contrasting personalities and, to some degree, ideologies.