The record 10-times French champions suffered a poor run in December but have enjoyed a spectacular recovery since then, collecting 17 points from seven league games and qualifying for the League Cup final and the French Cup last eight.
St Etienne, who have not featured in the Champions League for more than 30 years and are fifth in the Ligue 1 table with 44 points, could climb up to third on Saturday if they beat Nice, fourth on 45 (1900 GMT).
Third-ranked Olympique Marseille, who are on 46, take on lowly Troyes on Sunday (1600).
St Etienne signed three players on loan during the January transfer window and believe the tougher competition for places on the team has been the key to their 11-match unbeaten run in all competitions.
Unpredictable winger Yohan Mollo, who initially joined to replace Max-Alain Gradel while the Ivory Coast player was on international duty at the African Nations Cup, made an immediate impact, creating or scoring five goals.
Playmaker Mathieu Bodmer, who featured in only four league games at Paris St Germain during the first half of the season, shone in Tuesday's French Cup 3-2 victory over Lille.
"Competition is really helpful as we saw against Lille. Renaud Cohade and Mathieu Bodmer were back in the starting line-up and they set up one goal each," coach Christophe Galtier told reporters.
"Now, you have to score or create goals to start a game," added Galtier, who used to complain about the lack of high-level replacements when his side were struggling late last year.
The recruitment has also helped St Etienne to deal with playing in three domestic competitions, and they are the only side still involved in both cups.
"There were a lot of changes against Lille but we continue to have good results. It has shown that we're all involved in the project," winger Romain Hamouma said.
The only bad news ahead of this weekend's clash could be their poor record against Nice, who have won their last four league games at St Etienne.
The southern side, who had not been expected to fight for a European qualifying spot after finishing 13th last season, could be without midfielder and captain Didier Digard, who has joined a long list of injured players.
Leaders Paris St Germain visit lowly Stade Reims on Saturday (1600) while second-placed Olympique Lyon, who trail the capital club by three points, travel to Stade Brest on Sunday (1300).
Champions Montpellier are at home to Stade Rennes on Friday (1930), with the sides level on 41 points in sixth and seventh places.
- Sports & Recreation
- Mathieu Bodmer