"It's not interference... Ukrainian voters will reach their democratic decision in the run-off election as they see fit," spokesman Steffen Seibert said, referring to the April 21 poll.
"The contact person for the chancellor is always the head of government or president on the other side," Seibert said, adding the two leaders have urgent topics to discuss during their Berlin talks on April 12.
Germany has been a key broker in the conflict between Kiev and Russia-backed separatists in eastern Ukraine since Moscow annexed the Crimean peninsula in 2014, negotiating with France the now moribund Minsk peace accords.
But Seibert declined to be drawn on whether Merkel would meet Poroshenko's challenger, comedian Volodymyr Zelensky, ahead of the run-off vote.
Zelensky massively outshone Poroshenko in the first round on March 31, with supporters seeing him as a new face in a crusted-over political landscape.
But critics argue his programme is vague and doubt his ability to lead the crisis-wracked country.
Poroshenko backers credit him with binding Ukraine to the West, rebuilding the nation's army and launching significant reforms in Kiev.
The former oligarch is betting his experience will win over voters, although he stands accused of laxity in the fight against corruption.