The 21-year-old reigning individual pursuit world champion attacked out of a four-man breakaway with 30km to go, soloing across the finish more than half a minute ahead of fellow escapees Jan Bakelants (Ag2r La Mondiale) and third-placed Bert-Jan Lindeman (LottoNL-Jumbo).
“To win my first WorldTour race in Switzerland, with such a great crowd and with such a great team, is great,” Küng said. “I had a look at this stage before, a week ago. I knew it could be something for me. I like this kind of awful weather – rain and a little bit cold. I gave it a shot and in the end it turned out well. I had very good legs. It was amazing to take this win.”
Küng made his winning move on a descent off the penultimate climb of the day, leaving behind Bakelants, Lindeman and Team Katusha's Pavel Kochetkov as the quartet had a two-minute lead over the Sky-led peloton. Before the stage, Küng had mechanics fit his bike with a 54-tooth front chainring to take advantage of the downhill run to the finish.
“It is always very hard to know if you are going to win, but when I reached the top of the last (climb), I knew that it was looking quite likely,” Küng said.
The win on Friday was Küng's second this year after soloing to a win at the Volta Limburg last month.
“This is a big day for him and a big day for the team,” said BMC sport director Yvon Ledanois. “Stefan is a big talent, but he also listens well to the sport directors and to [performance manager] David Bailey. So it is a nice package. This is not a surprise for us. He is already a champion.”
Team director Max Sciandri said the weather played into the team's hands as rain poured down throughout the stage.
“I told the guys that 50 per cent of the peloton did not want to race and there were four-and-a-half hours of hard work ahead,” Sciandri said. “I said, 'Guys you have trained and prepared for this and you believe in it.' And Stefan was one of the guys on the early break duty. The first break went and then all the rest you saw on TV.”
Küng also earned the day's “most combative rider” honours for the effort.