Davydova and Ermakova amassed a total of 98.667 points, one full point more than second-placed Andrea Fuentes and Gemma Mengual of Spain, the European champions.
The "Two Anastasias," who have won the last three world championship duet titles, missed this year's European championships and the Spanish pair won in their absence.
"We both caught colds and had high fevers. It happened three or four days before our (scheduled) departure," Davydova said.
Flawless Russia complete clean sweep
"In all the competitions that we attended, they (the Spanish duet) didn't win. In Europe, the Spanish team is very strong but overall the Russian team is better."
Japan's Saho Harada and Emiko Suzuki were third overall with 96.500, followed by Chinese identical twins Jiang Tingting and Jiang Wenwen with 96.167.
China have risen steadily in the rankings since Japanese coach Masayo Imura was hired in 2006 and they are chasing their first Olympic medals in the sport in Beijing.
Russia won both duet and team at the last two Olympics. The free routine preliminary round is on Tuesday and free routine final on Wednesday. The final scores comprise 50 percent of the technical routine scores and 50 percent of the free routine final scores.
Teenage pair Jenna Randall and Olivia Allison continued to impress on the second day of their Olympic campaign with another solid performance.
However it was not enough for the pair - who are Team GB's first synchronised swimmers since Barcelona 1992 – to advance to Wednesday's free routine final.
Following their technical programme on Monday, which put them in fourteenth position overnight, the duo scored 44.667 in their quality free routine to maintain that placing.
Coming into the Beijing Olympics the pair had set a top sixteen target and were delighted to exceed that aim.
"Top 16 was our aim and we've bettered that with 14th less than a point from making the final. That's an amazing achievement," said Randall.
"We received three nines in that score and that means "excellent". In just 18 months we've gone from scoring sevens to nines and that's a big leap.
"These Games were all about experience and we now face a very intense and important four years going into London."
Allison said: "So much hard work has gone into getting ready for these Olympics. We've done a lot of cardio work for our stamina and weights for our strength, and the results are very exciting.
"We've sacrificed so much for Beijing but it's worth it. Education is on hold and training always comes first but days like today make it all worthwhile."
Russia defend synchro gold
Lights off for Spain