Davydenko's victory in Hamburg was his 15th career title and returned him into the top 10 at No. 9. After being injured for nearly the first four months, the Russian has won 15 of his last 18 matches.
In the quarterfinals, he will face Simone Bolelli of Italy, who defeated compatriot Fabio Fognini, 6-3, 6-4.
Second-seeded David Ferrer of Spain was out, however, upset by Nicolas Massu of Chile 4-6, 6-2, 6-4.
Massu, ranked 85th, came from 4-2 down in the third set to beat the former Umag finalist after 2 hours, 16 minutes. The win was Massu's first over a top-20 opponent since February 2008.
Massu will next meet seventh-seeded Andreas Seppi of Italy, who beat Frederico Gil of Portugal 7-5, 7-6 (5).
Fifth-seeded Juan Carlos Ferrero overcame Potito Starace of Italy 1-6, 6-3, 6-2 in what he hopes will become a nice coincidence.
Ferrero won his first ATP title in six years in April at Casablanca, and in both Casablanca and Umag, he defeated Christophe Rochus of Belgium in the first round, and Starace in the second round.
However, his quarterfinal opponent will be different. The former No. 1 will meet Maximo Gonzalez of Argentina, who ousted Oscar Hernandez of Spain 6-4, 6-4.
Fourth-seeded Jurgen Melzer of Austria beat Pablo Cuevas 6-4, 6-3, reversing a loss to the Uruguayan last week in Hamburg.
Melzer next plays Croatian Ivan Ljubicic, who stunned sixth-seeded Nicolas Almagro 7-5, 7-6 (2). Ljubicic converted his only break point at 6-5 in the first set. Almagro had eight break points but failed to convert any of them.
Keirrison open to Roma loanDavydenko rallies to advance to 2nd round
