Russians Natalia Ishchenko and Svetlana Romashina took the lead in synchronized swimming duets after the technical routine Sunday.
Ishchenko and Romashina, trying to land their country's fourth straight Olympic gold in the discipline, had 98.2 points to lead China's Huang Xuechen and Liu Ou by 2.1.
Spain's Ona Carbonell Ballestero and Andrea Fuentes Fache were third and Canada's Marie-Pier Boudreau Gagnon and Elise Marcotte stood fourth.
The duets free routine preliminaries are Monday. Scores will be added from the first two sessions and the top 12 teams will make the final Tuesday.
The team competition starts Thursday.
The United States failed to qualify a synchronized swimming team for the Olympics for the first time ever, but Mary Killman and Mariya Koroleva are representing the U.S. in duets. They were ninth Sunday with 87.9 points.
Russia has swept the synchronized swimming program in the last three Olympics, winning every gold in the team and duets competition since 2000.