Sandro's opening super-G run of 1:20.38 was only enough to see him occupy eighth place at the half-way stage as rapidly deteriorating conditions wrecked havoc on the field.
Although the early starters were blessed by sunshine on the Universiade piste, descending clouds and light snow soon made visibility hard. One notable victim was Andorra's Marc Oliveras, the silver medallist from Friday's super-G, who was among 14 athletes who failed to complete the course.
Winner of Friday's super-G gold medal, Tentori predictably posted the fastest time in the opening super-G leg, completing the technical course in a target time of 1:19.13 - over a second faster than Boner.
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But despite the improving visibility, the Italian struggled with the succession of gates on the slalom leg, and Tentori eventually finished more than 10 seconds in arrears in 22nd place after losing his balance and hitting the deck on a late gate.
Sandro, a 27-year-old distance learning student from Brig, put in a commanding slalom to secure the gold medal - a first for Switzerland - with a combined time of 2:04.70.
"The snow was both hard and very soft in patches but I was able to pick my line from following the tracks of the previous 22 skiers," said Sandro, who said he concentrated on his own run and tried to forget about his competitors.
Italy's Giulio Bosca took silver, trailing Boner over the two runs by 0.35 seconds after recording the second-best time in the super-G. The bronze medal - trailing Bosca by just one-hundredth of a second - went to Slovakia's Matej Falat, who rose from tenth place after the super-G.
Sunday's second medal event saw Russia's Alena Leonova put in a wondrous short program to snare her nation's seventeenth gold medal in the women's free skating in Granada.
Leonova won both rounds of the final on the ice rink of the Granada Igloo but it was the 24-year-old from St Petersburg's short program that earned her the most plaudits and set up her overall victory with 67.12 points.
Her second round free skate merited a further 115.73 points to give the Russian starlet a total haul of 182.85 points - more than 10 points higher than her nearest rival.
Mae Berenice Meite of France secured the silver with a total of 171.54 points (56.84/114.70) while Russia's Maria Artemieva begged the bronze with 170.24 points (62.11/108.13).
Leonova's gold was Russia's seventeenth of the 2015 Winter Universiade, while Artemieva's bronze increased Russia's tally to 47 medals in all competitions - a staggering 37 more than nearest rival, Kazakhstan.
Elsewhere, it was a good day in the Fuentenueva Pavilion for Great Britain's curlers, with the men beating Sweden 9-6 and the USA 5-4 and the women hammering the Republic of Korea 8-2. Tight encounters in the men's group stages saw hosts Spain beat the Czech Republic 7-6 and Switzerland edge USA 8-7.
The biggest win of the day came for the Canadian women's team, who pulverised Spain 16-2. The host's male team did not fare much better, losing 11-2 to Canada. Russia's women put in a similarly dominant performance with a resounding 12-2 victory over Norway, while the Russian men thrashed the Czech Republic 10-3.
In ice hockey, the Czech Republic practically decimated China 9-0 and Slovakia beat Spain 5-2 in the men's tournament, while Canada's women were beaten 3-1 by the Russian Federation. Monday's only medal event comes in snowboarding with the men's and women's slopestyle final.
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