The victory puts Northug 123 points behind overall leader Dario Cologna of Switzerland with six races to go. Cologna skipped Sunday's race.
Kowalczyk is virtually assured of winning her fourth overall title after extending her advantage to 581 points over second-place Therese Johaug of Norway.
SKI JUMPING: At Lahti, Finland, Richard Freitag of Germany won a large hill event in World Cup ski jumping Sunday while Gregor Schlierenzauer of Austria failed to clinch the overall title after finishing 15th.
This was Freitag's third career victory. He led after the first round and went 128.5 meters with the day's longest jump in the second and won with 274.2 points.
Anders Bardal finished second with 266.9 to keep the race for the overall title alive. Fellow Norwegian Anders Jacobsen and Germany's Richard Freund tied for third with 265.8.
FREESTYLE: At Voss, Norway, Jean Frederic Chapuis of France won the ski cross title for his first major victory Sunday, beating countryman Bastien Midol in the final at the freestyle world championships.
John Teller of the United States took the bronze. The fourth finalist, Jouni Pellinen of Finland, fell and failed to finish.
Chapuis has never won a World Cup event. His best finish was third place in Germany last year.
In the women's race, 20-year-old Fanny Smith of Switzerland beat World Cup champion Marielle Thompson of Canada. Ophelie David of France won the bronze medal.
SNOWBOARD: At Arosa, Switzerland, Rok Marguc of Slovenia and Hilde-Katrine Engeli of Norway earned their first career snowboard World Cup victories Sunday by winning parallel giant slalom events.
Marguc beat Matthew Morison of Canada in the men's final to follow up on the world championship gold he won in the parallel slalom in January. He had three previous World Cup podium finishes — the last one coming two years ago in Moscow.
In the women's race, Engeli beat Patrizia Kummer of Switzerland in the final.
SPEEDSKATING: At Heerenveen, Netherlands, Olympic champion Lee Sang-hwa of South Korea won the women's 500 meters Sunday at the speedskating World Cup to take the season discipline title.
Lee won in 37.77 seconds at the Thialf oval, beating Wang Beixing of China by one hundredth of a second. Thijsje Oenema of the Netherlands was third.
Wang ended the season third in the standings behind Jenny Wolf of Germany, who finished fourth Sunday.
Jan Smeekens, who won the 500 title Friday, underscored his dominance by winning his sixth straight race of the season in the distance, posting a time of 34.83 seconds.
Joji Kato of Japan was second and Smeekens' Netherlands teammate, Michel Mulder, was third.
Ireen Wust followed her blisteringly fast 3,000-meter victory Saturday with another win in the 1,500. Wust missed the first three 1,500 races of the World Cup season and ended third in the standings.
Marrit Leenstra of the Netherlands finished a disappointing eighth Sunday, but it was enough to ensure she won the overall season title.
Bart Swings, the Belgian former inline skater who has been one of the surprises of the speedskating season, won the men's 1,500. Zbigniew Brodka was second to top the season-ending World Cup standings.
BIATHLON: At Sochi, Russia, Dmitry Malyshko and Evgeny Ustyugov anchored the Russian team to a comeback win in the men's 4x7.5-kilometer relay at a biathlon World Cup meet Sunday.
Russia overcame one penalty loop and six missed targets to win in 1 hour, 9 minutes, 50.8 seconds — 38.3 seconds ahead of second-place Germany. The Czech Republic was third, its first podium this season.
World champion Norway, which was leading the discipline standings coming into the race, missed eight times and ran two penalty loops to finish fourth.
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