Twitter is digging its heels in against an encroaching Facebook in one of its biggest market, Japan. At a joint press conference in Tokyo on Wednesday, Twitter and Japan’s largest social network, mixi, announced a strategic partnership.

Twitter has always had a close relationship with Japan. Japanese was the first language that Twitter was translated into outside of English, and Japanese users are among the most prominent on the network. And earlier this year, Twitter chose Japan for the location of its first international country manager.

The partnership with mixi, as described on the Asiajin blog, will include developing new joint services and advertising products, and incorporating Twitter into mixi’s current platform.

Asiajin notes that part of this partnership is already in place on mixi’s website. Mixi Xmas, an official app from mixi, includes a Twitter button to the right of the button to send a message to mixi voice.

The two companies are going to focus on holiday-themed applications for now, which likely will not affect the majority of mixi users.

Twitter’s country manager for Japan, James Kondo, has this to say about the partnership:

“This is going to be an interesting case. We’re going to see what works and what doesn’t work, and we’re going to build on top of that as opposed to throwing out something that may not work.”

From the sounds of it, the companies are trying to create a joint front against Facebook. Twitter and Facebook have been in competition for some time in North America and other areas of the world, but it was only last year that Facebook started making real headway in Japan. And for mixi, formerly the largest social network in Japan and recently ousted from that position by Facebook, this could mean trouble unless they can team up with a company like Twitter and go on the offensive.