REDMOND, Wash.  Qi Lu knows as well as anyone just how difficult it is to take on Google.

For nearly a decade, Mr. Lu played a leading role in building Yahoo’s Internet search and advertising technologies. The effort was so important that Yahoo backed it with billions of dollars to acquire companies, hire armies of engineers and develop and run its own systems. Yet Yahoo fell further and further behind and many analysts said the company was simply outgunned by Google.

Mr. Lu, who is 47, left Yahoo 14 months ago, but now finds himself once again leading the charge against Google. This time, he is backed by a patron that vows to spend even more than Yahoo did on the mission: Microsoft.

“It’s an unfinished mission that I would like to work on,” he said.

The challenge for Mr. Lu and his team remains enormous, and success appears improbable. But since Steven A. Ballmer, Microsoft’s chief executive, tapped him to become president of the company’s online services division in December, Mr. Lu, a self-effacing engineer who is one of the most private and atypical executives in the upper ranks of the Internet industry, has earned the confidence of Microsoft’s troops and helped to bring a dose of optimism to a beaten-down team.

Possessing unusual stamina and a maniacal work ethic, he has pushed his team hard to give Microsoft an important victory. In nightly 9:30 meetings over several weeks, he leaned on his managers to find creative ways to structure a sweeping and complex partnership with Yahoo. The deal, signed in July, will give Microsoft something it has coveted for years: a vastly larger audience that will make Bing, its search engine, the runner-up to Google.