Russians Natalia Ishchenko and Svetlana Romashina kept their lead in synchronized swimming duets after the first free routine Monday.
Ishchenko and Romashina, trying to land Russia's fourth straight Olympic gold in the discipline, earned the highest score for the second day in a row. Their 98.6 points moved them to 196.8 -- 3.99 points ahead of China with Spain 4.21 back.
Canada's Marie-Pier Boudreau Gagnon and Elise Marcotte were fourth, and Mary Killman and Mariya Koroleva of the United States stood 10th.
Scores from the first two sessions were added together and only the top 12 pairs out of 24 made Tuesday's final.
The team competition starts Thursday. The U.S. doesn't have a team for the first time ever.
Russia has swept the synchronized swimming program in the last three Olympics, winning every gold in the team and duets competition since 2000.