Olympic champion Carl Lewis won a gold medal in the 100-meter dash after taking his heat in 10.29 seconds and reportedly declining to run in the final at the Seoul International Track and Field meet yesterday.

An official who asked that his name not be used said the 100 meters final was canceled because Lewis entered the meet on the condition he run only one race. Medals were awarded to the top three finishers in each heat.

Lewis, winner of four gold medals at the 1984 Olympics, arrived unannounced 11 hours before his race and ran in the third of three heats. American Darwin Cook won the first heat in 10.28 and Kimmo Saaristo of Finland won the second at 10.50.

Mary Decker Slaney won the women's 800 meters in 1:58.09. She finished about 10 yards ahead of Isabelle De Bruycker of Belgium, timed in 2:06.39. Ann McKenzie of Australia was third in 2:09.15 in the event at the Olympic Stadium.

In the men's triple jump, Los Angeles Olympic gold medalist Al Joyner of the United States was upset by West Germany's Wolfgang Zinser, who jumped 53 feet 11 inches to Joyner's 53-8 1/4.