The number 13 seed tumbled out 6-1 6-1 in 52 minutes to Davydenko, once ranked as high as three on the tour but now languishing at 50.
Elsewhere, 12th seed Marin Cilic came through a testing match against qualifier Sergiy Stakhovsky 6-3 4-6 6-4, while 15th seed Philipp Kohlschreiber bowed out 7-6 7-6 to America's Brian Baker.
There were straight sets wins for Andreas Seppi, Jesse Levine, Lu Yenhsun, Viktor Troicki, Florian Mayor, Pablo Andujar and Denis Istomin, while Switzerland's Stanislas Wawrinka won a three-set contest against France's Paul-Henri Mathieu.
Watch: Davydenko causes upset in Cincinnati.. More Videos
The opening day of the women's event saw Mona Barthel. Timea Babos, Anabel Medina Garrigues, Johanna Larsson, Julia Goerges, Eleni Daniilidou and Dominika Cibulkova all progress.
The biggest shock saw 14th seed Francesca Schiavone knocked out by fellow Italian Camila Giorgi, 6-1 6-3.