Thomas Wassberg of Sweden won the 50-kilometer race in the world Nordic ski championships yesterday--the classic event in cross-country competition--on the woodland trails of Oslo's Holmenkollen course.

Wassberg was timed in 2 hours 32 minutes .9 seconds. Yuri Burlakov of the Soviet Union was second in 2:32:34.3 and Lars-Erik Eriksen of Norway was third in 2:32:49.9, keeping alive Norway's streak of winning a medal in every event in these championships.

America's Tim Caldwell skied one of his finest-ever races to finish 14th. The American team competed without its No. 1 man, Bill Koch, who withdrew because of bronchitis . . . In Stockholm, World Cup leader Phil Mahre of the United States failed to reach the final and Sweden's Ingemar Stenmark fell and bruised his shoulder in an international parallel-slalom ski meet yesterday. The race, which was not a part of the World Cup series, was won by Lars-Goran Halvarsson of Sweden. Austria's Franz Gruber placed third and Bojan Krizaj of Yugoslavia was fourth . . . Marie Epple of West Germany kept up the pressure on her older sister Irene by winning the giant slalom in Aspen, Colo., yesterday, the only World Cup race for women in the United States this year. Marie had a combined time for two runs of 2:13.51 to edge Switzerland's Erika Hess by .16 seconds. Irene Epple was third in 2:14.16 and America's Karen Lancaster, whose previous best World Cup finish was a 20th place, came in fourth in 2:15.26.