Dave King won two 18-hole matches by 1-up counts yesterday, beating the defending champion and then the host club's best palyer, to reach today's 36-hole match play final of the District Amateur golf championship at Washingtonian's National course in Gaithersburg.

King, of Bethesda, faces madealist Wayne DeFrancesco of Cedar Crest, the 1978 tournament winner. King, who turned 40 last week, has reached the final only once before (1977) in 16 years of District Amateur play.

DeFrancesco, 23, defeated Ed Offterdinger of Bethesda, 4 and 2, in a semifinal match. DeFrancesco had beaten Gary Marlowe of Woodmont, 2 and 1, in the quarterfinals.

King, twice the Maryland Amateur winner (1969 and '71), beat Barry McCarty of Belle Haven, last year's District Amateur winner, in quarterfinal action with two birdies and an eagle.

In semifinal play, King downed 19-year-old Webb Heintzelman of Washingtonian, the area's top junior player in 1979 with six tournament vitories that summer.

Heintzelman coceded the match to King on the 593-yard, par-5 last hole. Heintzelman had tried to play his ball from the edge of the lake on his third stroke, but his further into the water. King, in trouble himself, pitched from a bunker to within inches of the cup and won the match. Heintzelman won his morning round over Harry Cain of Bethesda, 5 and 4.

Reaching the final continues a string of good golf by King, who shot 76 in his afternoon round. He won the Washington Area Amateur Championship by eight strokes in June, and finished second in the Kenridge Invitational in Charlottesville. He also finished one shot out of second place in the Eastern Amateur last week in Portsmouth.

This may be the last amateur final for DeFrancesco, who will attempt to earn his PGA playing card in August in Florida.

DeFrancesco, who finished his round at even par 70, needed only 16 holes to close out Offterdinger. DeFrancesco was 4 up after he birdied that 13th. He bogeyed the next two holes to fall to 2 up and halved the 16th with a part 4 on the 406-yard hole to win the match.

Offterdinger reached the semifinals with a 2-and-1 victory over Rodney Stanley of Chantilly.