District of Columbia amateur fighter Lyndon Walker, the American Boxing Federation featherweight champion, was a confident spectator at last night's preliminaries of the inaugural D.C. Mayor's Cup Invitational at Howard University.

"Nothing to worry about," Walker said. "Everybody in my weight division I've beaten before."

The Washington team is competing against teams from Fort Bragg, N.C., Philadelphia, New York, Baltimore, Virginia Beach, Paterson, N.J., and Plainfield, N.J.

"I feel Washington has a strong team," said welterweight Darryl Lattimore. "We'll have the trophy come Saturday."

Walker, a Spingarn High School student with a 121-14 record, has fought in the Golden Gloves. A teammate, light flyweight James Harris, also of Spingarn, has never lost an international fight. Lattimore (43-5) has been boxing only three years, but fought in a qualifying bout for the 1984 Olympics.

While the three boxers await what they consider easy bouts in the semifinals, only one District boxer won last night. That was Anthony Ross, at 139 pounds, with a unanimous decision over Derrick Wilson of Fort Bragg.

At 112 pounds, Gregory Vicente of Fort Bragg gained a unanimous decision in his three-round match with Anthony Wilson of Washington. In the third round, Wilson connected with a few hard left hooks but became tired. At 156 pounds, Keith Grant, Philadelphia, stopped Sylvester Barnes of Washington in the second round.