Dinamo Zagreb have taken a slender lead over Olympiacos into half time thanks to Armin Hodzic's 21st-minute header.
Konstantinos Fortounis had the opening chance of the game after six minutes, as he rounded Zagreb midfielder Paulo Machado before firing narrowly wide.
Hernani, who started the tie in favour of Felipe Pardo, then found some room for a strike but shot straight at Dinamo keeper Eduardo despite having Omar Elabdellaoui in space.
The Portuguese winger missed another opportunity in the 16th minute, heading just wide from Arthur Masuaku's cross with Eduardo stranded.
It was Dinamo who struck first in the opening half as Josip Pivaric went beyond Dimitris Siovas before crossing for Hodzic, who nodded his side into the lead with 21 minutes of the tie expired.
Seba went close for Marco Silva's side with five minutes left in the half, as he latched on to Hernani's through-ball but saw his effort turned behind by Eduardo.
