The popular race was originally a competition for army ski teams in 1943, an endurance test for World War II border patrols high in southwestern Switzerland.
It was revived in 1984 and opened up to civilians and foreign teams of three as well over the years, ballooning to some 4,200 participants and forcing the Swiss army organisers to split it up in to several races.
The first amateur teams of three started arriving Thursday at their destination at 9:00 a.m. (0700 GMT), the organisers said.
Some of them tackled the shorter 26 kilometre race from Arolla in six and a half hours, close to the record time of six hours and 18 minutes set in 2006 over the full course.
Another 2,200 specialists were due to take part in the elite race overnight into Saturday, climbing on tour skis and by foot over the steepest and rockiest portions, then donning their skis again for some respite downhill.
Between Zermatt and Verbier they confront the highest peak at the Riedmatten pass as well as three more walls of snow and rock.
Yet crowds still gathered on the isolated high slopes at the Rosablanche pass despite the early hour on Thursday to cheer on the weary patrols to the sound of cow bells hauled up to the final slope for the occasion.
"I'm happy, the hardest is over," wheezed Lara Allet, 33, as she waited for her teammates to join her at the top of the pass, before they donned their skis a last 10 kilometre stretch to Verbier.
"I'm burnt out," another skier told AFP as he wiped sweat from his brow despite the bitter cold.
More than half of the 1,400 patrols of three are civilians nowadays and the popularity of the event is such that the top race from Friday into Saturday is broadcast live on Swiss television, despite the huge logistical effort it involves.
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