Yahoo has announced a partnership with rival Google to start testing Google-generated ads in its website during a period of two weeks. A starting date was not specified.

The test will involve placing Google search ads to about 3% of the queries made in the U.S. through Yahoo’s search engine. After the two-week trial, Yahoo will evaluate whether it generates more revenue by outsourcing ads from Google or through its own ad sales system.

Yahoo’s move is clearly an attempt to show Microsoft that it has other options besides taking its initial bid of $42 billion (or $31a share). Over the weekend, Microsoft set a three-week deadline for Yahoo to begin negotiations, a demand Yahoo declined by restating its contention the offer undervalues the company.

Microsoft fired back saying that a deal between Yahoo and Google would make the search market less competitive.

Indeed, according to ComScore’s MediaMetrix data, Yahoo’s market share continues to drop and accounts to 21.6% in February. In the same way, Microsoft’s share is also down to 9.6% for the month of February. In the meantime, Google’s share has grown to 59.2% for the same time period. If a Google-Yahoo alliance occurred, it would represent a total of about 81% of the total search market.