Roscoe Tanner and Bob Hewitt moved into the semifinals of the doubles competition in the Washington Star International Tennis Championships yesterday in a match in which the sets were as identical as the opponents.

They defeated Tim and Tom Gullikson, identical twins from Onalaska, Wis., 6-4, 6-4. In both sets, the only player whose serve was broken was Tim Gullikson.

Tanner and Hewitt broke the serve in the third game of the first set to go ahead two games to one. All players held serve the rest of the match.

In the second set, some quick exchanges at the net between Gulliksons and Hewitt kept things lively, but again each player managed to hold serve until the ninth game. With Tim serving. Tanner and Hewitt returned some towering lobs and broke Tim to go up, 5-4, Hewitt served for the match.

Hewitt, a South African with salt-and-pepper beard and balding head, is one of the leading doubles players in the history of tennis. Currently ranked No. 1 in doubles on the Colgate Grand Prix circuit, he has won doubles at Wimbleton four times, the South African Open four times and the Australian Open twice.

Tanner had an excellent doubles year last year, winning five events, although none with Hewitt. The Tanner-Hewitt combination reached the final of the Western Open last week in Cincinnati. Their regular doubles partners are competing in team tennis this summer.

The Gulliksons (and if it weren't already hard enough to tell them apart, their names were mis-spelled on the score board) reached the doubles final at Newport and the semifinals at Hampton.

Some 20 protesters picketed the gate of the Washington Tennis Stadium from 4 p.m. to 6:20, demonstrating against the tournament's South African players, including Hewitt, a native Australian who emigrated to Johannesburg in 1964 and plays Davis Cup for South Africa. The pickets carried signs and passed out leaflets but were orderly and caused no incidents.