Google continues to hire away top Microsoft talent. But this time, Microsoft is fighting back.

On Tuesday, Google announced plans to open a product research-and-development center in China, and said it was appointing former Microsoft vice president Kai-Fu Lee to head the operation.

On Wednesday, Microsoft announced it was filing a lawsuit against Lee and Google, claiming breach of both employee confidentiality and non-compete agreement.

"We are asking the Court to require Dr. Lee and Google to honor the confidentiality and non-competition agreements he signed when he began working for Microsoft," said Microsoft via a corporate statement.

"Creating intellectual property is the essence of what we do at Microsoft, and we have a responsibility to our employees and our shareholders to protect our intellectual property," the Microsoft statement continued. "As a senior executive, Dr. Lee has direct knowledge of Microsoft's trade secrets concerning search technologies and China business strategies. He has accepted a position focused on the same set of technologies and strategies for a direct competitor in egregious violation of his explicit contractual obligations."

Lee, a seven-year Microsoft veteran, served most recently as corporate vice president in charge of Microsoft's natural interactive services division. That unit focused on simplifying user interfaces, especially via speech and natural-language technologies. Lee also was the founder of Microsoft Research Asia, and held positions with Silicon Graphics and Apple Computer prior to joining Microsoft.

One of the projects upon which Lee worked during his time at Microsoft was known as the NUI (natural user interface) platform. At one point, the NUI platform — designed provide users with "rich interaction" (speech, handwriting, natural language and even machine learning)— was set to be incorporated into Longhorn.

Lee also was instrumental in developing and bringing to market Microsoft Speech Server.

Lee is not the first high-profile Microsoft executive to go to Microsoft rival Google. Late last year, one of Microsoft's key Windows architects, Marc Lucovsky, defected to Google. Google has not made public Lucovsky's title and has declined to provide more details on his role.

But Lucovsky was one of a handful of "Distinguished Engineers" at Microsoft. He was credited as one of the core dozen engineers that came from Digital Equipment Corp. to Microsoft and built the Windows NT operating system. He was charged with building the Windows NT executive, kernel, Win32 run-time and other key elements of the operating system. NT was the precursor to Windows Server.

His appointment to Google fueled speculation that Google is developing a "GoogleOS" operating system.

Other mid-level Microsoft executives have joined Google over the past couple of years, as well. And Google opened a product-development office in Kirkland, Wash., late last year. Some industry watchers speculated that Google did so in order to attract more hires from Microsoft, which is headquartered in nearby Redmond, Wash.

It's the mid-level product and program managers whom Microsoft and other tech companies should guard most jealously, said Directions on Microsoft analyst Michael Cherry.

"While a lot of people make a big thing about the executives and senior vice presidents that leave, these people rarely ship software," Cherry said. "I think it is a bigger issue when the group program managers and program managers with ten plus years of experience silently leave. No one mourns their departure, but these are the people that take the grandiose architectures and wild-eyed visions and actually make them into products that people can use—and do so in a timely manner.

"The loss of these silent but hard working employees who keep the teams working together may have a bigger effect on the schedules of products like Yukon and Longhorn, and have a bigger long term impact on the company than any of the growing number of VPs and visionary architects," Cherry added.