They compiled a preliminary round total of 99.001 points, comprising 50 percent of their marks from the technical routine and 50 percent of Tuesday's 3-1/2 minute preliminary free routine score.
Japan's Miya Tachibana and Miho Takeda, silver medallists at the Sydney Olympics and the 2003 world championships in Barcelona, qualified second, suggesting another silver could well be theirs, with 98.000 points.
Alison Bartosik and Anna Kozlova of the U.S. were third with 96.751 points as Russian-born Kozlova, fourth at the last Olympics with former duet partner Tuesday Middaugh, sought to move up into the Olympic medal reckoning.
Russian star duo make fine start
Bartosik and Kozlova were fourth in the 2003 world championships in Barcelona behind bronze medallists Gemma Mengual and Paola Tirados of Spain but gained an edge over them this time.
The Spanish pair were fourth on 96.084 ahead of Virginie Dedieu and Laure Thibaud of France (95.251). Dedieu won a bronze medal at the 2000 Olympics when her partner was Myriam Lignot.
The top 12 of 24 duets qualified for Wednesday's free routine final.
Synchronised swimming entered the Olympic programme at the 1984 Los Angeles Games. The duet will be followed by the team event, in which Russia will defend the title.
USA's Artistic Swimming performance at London 2012
Russia’s near-perfect synchronised swimming routine at London 2012