A martial arts master and a second man have been convicted of beating up a Menlo Park man who had struck up a conversation with the judo expert about their shared passion, authorities said.

Matthew Walker, 25, of Tacoma, Wash., who won his category in the 2009 President's Cup tournament held in June by USA Judo, pleaded no contest earlier this month to felony assault, said Steve Wagstaffe, San Mateo County's chief deputy district attorney.

Walker was sentenced Feb. 2 to six months in county jail and placed on three years of probation.

A co-defendant, Robert Davis, 22, of Redwood City pleaded no contest Thursday to the same charge with an agreement that the case would be reduced to a misdemeanor halfway through his probation, Wagstaffe said. Davis faces as much as three months in jail when he is sentenced April 27.

The incident unfolded at the Menlo Park Caltrain station Sept. 29 when the 25-year-old victim and his 23-year-old friend from New York met Walker and Davis after they all got off a train headed home from a Giants game.

The Menlo Park man approached Walker - who was wearing a judo jacket and is 6 feet 3 inches tall and 275 pounds - and they struck up a conversation, prosecutors said. The man had some training in aikido, prosecutors said.

The two engaged in some horseplay, but the victim asked Walker to stop when it became too rough, authorities said. Both Walker and Davis then attacked the men and beat them severely, authorities said.

The Menlo Park man ended up with a broken leg and facial wounds. The New York victim was knocked out.

Menlo Park police saw one of the victims wandering in the street and began an investigation. They soon spotted Walker and Davis walking through a parking lot nearby. Officers saw Walker tossing what turned out to be one of the victim's cell phones in a garbage can, prosecutors said.