In Le Brassus, Switzerland, Bill Koch, who had astounded the world at Innsbruck, Austria, six years ago when he won an Olympic silver medal in cross country skiing, sprang another surprise when he won yesterday's World Nordic Cup 15-kilometer race. It marked the first time an American had won a world cup cross country race.

"This is my most satisfying result since that second place at the Olympics," the 27-year-old American said after he had outdueled Sweden's reigning Olympic champion Thomas Wassberg in a battle for first place.

In one of the fastest 15 kilometers races ever, Koch clocked 40 minutes 3.0 second, followed by Wassberg in 40.13.2 and two other Swedes, Benny Kohlberg in 40:23.9 and Thomas Eriksson in 40:31.7 . . . In Kitzbuehel, Austria, Canadian Steve Podborski established himself as a clear favorite in the forthcoming World Alpine skiing championship's downhill race and American Phil Mahre made a big stride toward winning the men's overall World Cup standings.

Following a second place in the first of two consecutive downhills Friday, Podborski skied to victory yesterday. His winning time was 1 minute 57.24 seconds, only four-hundredths of a second slower than Friday's winner, Harti Weirather of Austria. Mahre, of Yakima, Wash., in beating some of the downhill specialists, finished 14th in 1:58.98 for two Cup points.