Jurgen Klopp's side lost 3-0 to Bundesliga strugglers Hamburg at the weekend, but the midfielder feels the team will have deliberately been saving themselves for Tuesday's clash
Dortmund slumped to a shock 3-0 defeat against Hamburg in the Bundesliga over the weekend which saw them fall 20 points behind league leaders Bayern Munich.
But Tymoshchuk, who was part of the Bayern side which lifted Europe's top prize at the expense of Dortmund last May, believes Jurgen Klopp's men will be all-the-more focused on getting a positive result in Russia after succumbing to their sixth league defeat of the season.
"That happens when a team is preparing for the Champions League," he told reporters when asked about BVB's defeat.
"In the Bundesliga everything is clear, Bayern can be called champions. Dortmund are also focused on the Champions League, so I would not be surprised if they specifically left something to spare before meeting us."
Tymoshchuk spent five years at Bayern after earlier stints with Zenit and Shakhtar Donetsk, and the Ukraine international hopes his experiences of facing Dortmund will prove valuable to coach Luciano Spalletti in the build-up to Tuesday's game.
"If the coach needs my help in preparing for the match, I am willing to assist in any way I can," said the 34-year-old.
"I think everyone understands the overall picture. There are scouts who have specially watched Borussia's games. In the last few days we have thoroughly studied our opponents."
Zenit's last competitive game was the 4-1 group-stage defeat to Austria Vienna, but Tymoshchuk insists his side have been working tirelessly in training to recover full fitness.
"We worked so hard in training that we did not have time to shave," he added. "Two training sessions a day. We are very well prepared, the sessions were well organised and the team worked on high-performance tasks set by the coaching staff. So we are ready for tomorrow's match."
Zenit's last 16 first-leg clash with Dortmund kicks off at 18:00 CET on Tuesday.