Defending champion Johann Olav Koss of Norway set a world record for 5,000 meters yesterday as he took a commanding lead in the overall standings midway through the speedskating world championchips in Heerenveen, Netherlands.

But the biggest surprise of the day was a second-place finish by American Nathaniel Mills of Northfield, Ill., in the 500 meters. Given virtually no chance of finishing among the top three, he skated his best-ever 500, 37.87 seconds. Peter Adeberg of Germany also set a personal record in winning in 37.52.

Koss was timed in 6 minutes 41.73 seconds, cutting 1.86 seconds off the world record set in 1987 by Geir Karlstadt, also a Norwegian.

"Realistically, the championship is over," 1989 world champion Leo Visser of the Netherlands said of Koss's performance.

Koss has 78.633 points, with Roberto Sighel of Italy second with 79.174 after surprisingly strong showings in both the 500 and 5,000.

Third and fourth are Visser with 79.844 and Bart Veldkamp of the Netherlands with 80.026. Olympic champion Tomas Gustafson of Sweden is fifth with 80.110. . . .

Carole Merle of France fought off a German challenge to capture a women's super-giant slalom World Cup race in Garmisch-Partenkirchen, Germany.

Merle, the silver medalist in the event at the world championships last month, finished in 1 minute 26.14 seconds on the Kandahar course. Karin Dedler of Germany was second in 1:26.67 and teammate Michaela Gerg third in 1:26.80. . . .

Bjorn Dahlie of Norway won the 15-kilometer freestyle event in the world nordic championships in Val di Fiemme, Italy, and Sweden's Gunde Svan set an all-time record for medals.

Dahlie's effort on the tough uphill stretches toward the end keyed an 8.4-second victory over Svan.

Svan's silver medal gave him a total of 10 world and Olympic medals, matching the total of fellow Swede Sixten Jernberg and Veikko Hakulinen of Finland, both of whom retired in the early 1960s.

But Svan, who captured the opening 30-kilometer classic-style event on Thursday, has the most golds, seven.

Norway had a great day on the trails as Fred-Boerre Lundberg won the nordic combined championship two hours after Dahlie's triumph. . . .

Defending champion Gustav Weder drove Switzerland I to a narrow lead in the two-man bobsledding world championships in Altenberg, Germany, with the final two runs set for today.

Weder and co-driver Curdin Morell had an aggregate time of 1 minute 54.77 seconds, giving them a lead of .06 seconds over Germany II, driven by Rudi Lochner and Markus Zimmermann.