Second half goals from Robert Mak and Giuliano were enough to hand the visitors a hard-fought three points
Second half goals from Robert Mak and Giuliano handed Dundalk their first defeat in the Europa League group stage, as they were beaten 2-1 at Tallaght Stadium.
Robbie Benson had put the home side ahead just before the hour mark when his swerving effort from distance beat Yuri Lodygin in the Zenit goal.
But as the game wore on, Dundalk began to show signs of tiredness, with tonight's clash constituting their fifth game in 12 days. Concentration levels dropped and the visiting Russians were able to pick apart the Dundalk defence.
A backpass from Chris Shiels to Gabriel Sava was not properly cleared and the high ball fell in favour of Giuliano who played a slick pass to Domenico Criscito who in turn found Mak and from there it took a neat side foot finish.
Dundalk heads went and they were to drop even further when Giuliano struck to put the visitors ahead.
Axel Witsel was the architect this time as he picked up the ball on the right hand side of the box and fed Giliuano. Witsel drove toward the byline and then returned the ball to Giliuano who finished low into the net.
Dundalk, to their credit, made a promising start to the game. Ronan Finn carved out the first real chance of the evening. David McMillan launched a high long ball into the Zenit half after receiving a short lay-off in midfield.
Finn gave chase, burning Zenit centre back Nicolas Lombaerts for pace. His touch brought him into the box and he kept his drive low but unfortunately dragged it just wide of the far post.
The game continued with a steady balance where both sides had chances to push forward. The pace displayed by Finn, Benson and Daryl Horgan looked enough to suggest that Stephen Kenny's side had set up to play on the counter-attack.
Zenit St Petersburg then grew into the game, holding possession for long periods as Dundalk sat back. They pressed man-to-man but kept a compact shape and began to frustrate the Russians.
Their penetration came from the right wing, full-back Aleksander Anyukov flashing a shot across the face of goal toward the end of the first half which was about as close as they got to breaking the deadlock.
Giuliano looked lively from the outset, but his shot from outside the box after 13 minutes flew widely over the bar.
Dundalk remained composed throughout the opening 45, on more than one occasion did Sava opt to go short on the kick-out and allowed Dundalk to play out from the back.
Their chances came then after a period of sustained pressure.
Andy Boyle went close again after Dundalk won a corner from a cross blocked down the right hand side. A lofted cross wasn't fully cleared and captain Boyle was on hand to nod it just over the bar.
The crowd lifted after that moment, chanting continuously throughout the opening 45 minutes. Their cheers were further amplified when Oleg Shatov went into the book for what referee Miroslav Zelinka judged to be a dive.
Brian Gartland's offside header shortly before the break further exposed Zenit's inability to fully clear the high ball, which as Dundalk have shown in previous group games they are fully capable of exploiting.
Dane Massey's second half header from Horgan's freekick came off the post as Dundalk went in search of a second that would have left them in a far less precarious position.
But with their efforts came the added strain brought on by a jammed fixture schedule and even in the final minutes their defensive efforts looked slightly laboured.
Gartland conceded a late penalty after fouling Mauricio but the Brazilian couldn't convert from the spot as Sava, perhaps, made amends for his earlier mistake.
The game by this stage felt past the Premier Division champions and a late freekick from Sava to pump into the box didn't beat the first man, further evidence of their tiring legs.
Take nothing away from Stephen Kenny's side, they went ahead but were beaten by a better side and their run of games proved too much for the 'home' side.
The focus now returns to a domestic title defence and a chance at an incredible three-in-a-row.