A tight match between Jesualdo Ferreira's Greens and in-form Atromitos was settled in the 72nd minute when former Arsenal winger Quincy, on loan from Al-Saad, drilled home Sebastian Leto's knock-down to seal the three points.
It was the third 1-0 victory in succession for Ferreira's team, who are now two points clear of Olympiakos after collecting 42 points from 17 matches, one match less than the champions.
"Sometimes external factors affect you and Olympiakos' defeat did actually put added pressure on us; but we dominated the second half and showed that we want to win and we have to keep doing that," Ferreria told reporters.
"Nobody has won the title yet and there are plenty matches to come, though. We must be realistic and keep ourselves grounded."
Ernesto Valverde's Olympiakos created a host of opportunities but failed to break down a stubborn Skoda Xanthi side and the Pireaus club paid the price as Panagiotis Vlachodimos's well-taken goal seven minutes after half-time proved enough to secure a 1-0 victory.
Without injured strikers Kevin Mirallas and Rafik Djebbour, who have scored 20 goals this season, Olympiakos struggled in front of goal.
"The truth is that we were missing two very important attacking players but even in their absence we created enough opportunities to score and we should have managed without them," said Valverde.
"It's strange to leave with no points after giving out best performance of 2012 overall. The players were nervy after going a goal down and that is understandable given that we can't afford to lose many more points."
Fourth-placed AEK Athens were held to a 1-1 draw in Crete by Ergotelis, a result which gave PAOK Salonica a chance to gain ground.
Laszlo Boloni's men did just that by picking up a 1-0 home victory over Panionios, with Stefanos Athanasiadis's seventh goal of the season decisive at the Toumba Stadium.
That result set up an enticing match between AEK and PAOK at the Olympic Stadium next weekend with just two points separating the two teams.
