SAN FRANCISCO — Microsoft finally persuaded Yahoo to surrender control of the Internet's second most popular search engine and join it in a daunting battle — taking on the overwhelming dominance of Google in the online advertising market.

A 10-year deal announced yesterday gives Microsoft its best shot yet to show its new search technology, Bing, is as good as or better than Google's. Microsoft also hopes to use Yahoo to divert sales from Google, which generates more than $20 billion a year from ads.

Gaining access to Yahoo's audience would instantly more than triple Bing's U.S. market share to 28 percent. That's still a far cry from the remarkable 65 percent of U.S. searches handled by Google, according to the research firm comScore Inc.

By joining forces, Microsoft and Yahoo are betting they will be able to focus on their respective strengths. By turning over responsibility for search technology to Microsoft, Yahoo can concentrate on sales of billboard-style advertising on the Web — and figuring out how to keep luring traffic to its Web sites, which already attract more than 570 million people worldwide every month.

While the agreement shapes up as a potential boon for Microsoft, it was greeted in the stock market as a letdown for Yahoo. Just 14 months ago, Microsoft dangled $9 billion in front of Yahoo in an attempt to forge a search advertising partnership, only to be rebuffed. Yahoo also had turned down Microsoft's $47.5 billion offer to purchase the entire company.

Yahoo has been struggling so badly since then that Microsoft isn't paying any money in advance. Instead, it will give Yahoo 88 percent of the search ad sales made on its Web site, above the usual commission of 70 to 80 percent.

By spending less on its own search technology, Yahoo expects to boost its annual operating profit by about $500 million — but not until 2012, when the two companies expect to have all the pieces of a complex technological puzzle in place.

"I think a lot of people are kind of looking at the numbers and seeing a lot of question marks where they expected to see exclamation points," said Scott Kessler, a Standard & Poor's equity analyst.