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Carlo Ancelotti's side dominated the opening minutes at Stade Geoffroy-Guichard and went ahead after nine minutes. Javier Pastore received the ball inside the Saint-Etienne area and chipped Stephane Ruffier.
Ten minutes later PSG scored their second goal of the evening from the penalty spot.
Ruffier brought down Ezequiel Lavezzi and Zlatan Ibrahimovic made no mistake from 12 yards. The Swedish striker scored at the first time of asking, but was asked to re-take it; however, the former AC Milan man retained his cool to double his side's advantage.
However, Saint-Etienne came back strongly in the contest thereafter and halved the deficit eight minutes from time when Alex diverted Pierre-Emerick Aubameyang's right-wing cross into his own net while under pressure from Brandao.
The hosts stepped up their game in the second half and created two chances in the space of a minute around the hour mark.
That miss almost came back to haunt Saint-Etienne a couple of minutes later when Pastore shot over the crossbar following some disappointing defending from the home side.
The result sees PSG go five points clear at the top of the Ligue 1 table, while Saint-Etienne remain fourth.