Russia's Anastasia Davydova and Anastasia Ermikova lead the way after the first phase of the synchronised swimming duet competition.
Anastasia Davydova and Anastasia Ermakova
The duo compiled 98.833 points in the technical routine which involves the execution of required elements.
Japan's Miya Tachibana and Miho Takeda, runners-up at the Sydney Olympics, were second again with 98.000 points.
Alison Bartosik and Anna Kozlova of the USA were third, just ahead of Spain's Gemma Mengual and Paola Tirados.
Davydova and Ermikova are looking to inherit the title won by fellow Russians Olga Brusnikova and Maria Kisseleva at the 2000 Sydney Games.
Russian-born Kozlova, fourth at the last Olympics with a different partner and fourth at the worlds with Bartosik, competed at the 1992 Games for the Commonwealth of Independent States.
The top 12 of 24 duets qualify for Wednesday's free routine final from a combination of 50 percent of the total technical routine scores and 50 percent of Tuesday's preliminary free routine marks.
The duet will be followed by the team
event, in which Russia will defend the title.