Gisin clocked 1min 42.37sec down the 2.7km-long course in brilliant sunshine at Rosa Khutor to finish 0.19sec ahead of teammate Lara Gut, with Sweden's Kajsa Kling in third at 0.25sec.
Gisin and Gut will be joined in Wednesday's downhill by Fabienne Suter, who won Friday's training, and Marianne Kaufmann-Abderhalden.
Nadja Jnglin-Kamer was left in tears after crashing out to kiss goodbye to any chance she had of making the Swiss team.
"We had qualifications, and it was like a race. I went down without any questions," said Gisin. "The winner of yesterday's training got a spot, and the winner of today got a spot. It's the rules of the Olympics.
"It's crazy that you have more competition on a usual World Cup race than at the Olympics because some of the top 15 even will not race. But those are the rules and we know it.
"It's always kind of tough for the countries with a lot of good athletes. It's the same for American track and field, I think."
Gisin added: "It was a lot of pressure, but for me it was good that I could handle the pressure, that gives me a lot of confidence. I'm now looking forward to doing one easy training!"
There were four Americans in the top eight, 21-year-old Jacqueline Wiles in fourth from Laurenne Ross and Stacey Cook, with veteran Julia Mancuso in eighth at 1.38sec.
A raft of gold medal favourites sat out Saturday's training, including Austria's Anna Fenninger — winner of Thursday's run, Slovenian Tina Maze, Germany's Maria Hoefl-Riesch and Liechtenstein's Tina Weirather.