Switzerland, two times a winner of the women's silver medal, edged Denmark 7-6 before downing South Korea 8-6.
The Swiss have a 3-0 record and the only other unbeaten team in the tournament is Canada at 2-0. The Canadians have medaled in all four of the women's Olympic tournaments, taking a silver four years ago in Vancouver.
A nine-game round-robin tournament will be played in Sochi, at the end of which the top four teams will compete in the semifinals.
Russia is alone in third place at 2-1 after defeating the United States 9-7 and falling to China 7-5. Two-time defending champion Sweden, South Korea, Japan, Britain and China are all at 1-1. The United States and Denmark are winless in three games.