Aleksandr Kerzhakov scored just four minutes after his half-time introduction to secure the win for FC Zenit in Farum, leaving FC Nordsjælland up against it ahead of next week's return in Russia.
Both contenders were in the group stage last season, though Luciano Spalletti's side now seem the more likely to reproduce that showing this term thanks to Kerzhakov's intervention in Denmark. The hosts stuck rigidly to their passing, possession-based game, but were unable to find an equaliser to take into next Wednesday's return.
It was home keeper Martin Hansen who was the busier of the two custodians in the first period as Andrey Arshavin and Hulk tested him out with a series of efforts. Arshavin proved particularly dangerous, threatening the Nordsjælland defence with a number of slaloming runs.
The former Arsenal FC man was unable to find the breakthrough, however, and was replaced by Kerzhakov at the interval. Spalletti was vindicated shortly after as the substitute provided the finishing touch to a devastating counterattack to clinch a potentially crucial away goal.
The hosts came back into it and tested out Yuri Lodygin through Mario Tičinović and Kim Aabech, while Jens Stryger Larsen missed the target with a good effort. Zenit proved the stronger side in the end, though, and finished with a deserved lead to take back to St Petersburg.