The sides will meet in the last 16 of the competition, with the first leg taking place on February 25 at the Petrovsky Stadium.
Dortmund reached last season's final, ultimately suffering defeat to Bayern Munich, yet Tymoshchuk feels Jurgen Klopp's side are not the force they were.
The former Bayern midfielder believes Dortmund's current form, which has seen the German side win just one of their last five league matches, makes the tie winnable for Zenit.
"Borussia are third in the Bundesliga. In my opinion they are not in the best form right now," he said.
"They collected points against other leaders of the league and in the UEFA Champions League, but it's not the same team that reached the final last season.
"They were much stronger at that time."
