On the first of the three-day final leg of the World Cup tour, Svahn showed she was a class above the early competitors, completing the course over two minutes quicker than the previous fastest, Kaidy Kaasiku of Estonia, in a time of 29:04.4.
But it would be a nervous wait for Svahn, given that all the main competitors were still out on the course, including Karlsson, who beat Svahn to gold in the same event in Lahti earlier this month, and in the 50km mass start in Oslo last weekend.
One by one, more rivals finished without surpassing the individual sprint gold medallist from Milan-Cortina, who had just enough to cling on, as Karlsson finished 1.4 seconds down to take silver. Norway's Heidi Weng finished third, 22.1 seconds behind the winner.
Moa Ilar finished eighth, while fellow Swedes Jonna Sundling, Emma Ribom and Maja Dahlqvist all finished in the first 17. Ilar remains second in the overall World Cup standings but was slower than leader Jessie Diggins of the USA, who confirmed her third World Cup title on home snow.
In the men's event, Norway's Johannes Hosflot Klaebo returned from the concussion that forced him to miss Oslo, and immediately returned to the top of the podium, completing the 10km in a time of 26:44.5.
The first five home were all Norwegians, as Andreas Fjorden Ree took silver, ahead of Mattis Stenshagen. Saturday (21st) sees the men's and women's sprint freestyle races.
