Menez, who lost his status as a regular starter this season, fired past Benoit Costil in the 57th minute to put PSG on 64 points with seven games left with Olympique Marseille on 57 after beating Girondins Bordeaux on Friday.
Zlatan Ibrahimovic headed home the second in stoppage time from David Beckham's chipped ball.
Rennes, who were fourth entering last December's break, are 10th on 42 points.
"It was a very complicated game," France defender Mamadou Sakho told French TV channel Canal Plus.
"But we have a winning mentality - just see the team's reaction after the second goal. It shows our desire to go get this title."
Coach Carlo Ancelotti fielded a largely reserve team four days before a Champions League quarter-final return leg at Barcelona.
Midfielder Javier Pastore as well as fellow Argentine Ezequiel Lavezzi, fullback Christophe Jallet and midfielder Beckham all started on the bench while Brazil centre back Thiago Silva was not even on the team sheet.
Striker Ibrahimovic was one of only four players who started against Barca at the Parc des Princes last Tuesday when PSG snatched a 2-2 draw in the first leg.
Keeper Salvatore Sirigu stretched on to his left to deny Sadio Diallo's low shot in the 14th minute as PSG got off to a sluggish start.
Rennes, however, looked too shy to benefit from PSG's apathy in a dull first half.
Menez, who has started most of PSG's important games on the bench recently, was keen to make a point, showing the visiting fans his name on the back of his shirt after coolly slotting home following a dazzling run into the Rennes defence.
He still had a sulky look on his face when he was substituted by Pastore 15 minutes from time.
Beckham and Lavezzi, who came in for Marco Verratti and Kevin Gameiro respectively, also had some play time ahead of the Nou Camp clash.
Beckham attempted a chip with the outside of his boot in the 93rd, and Ibrahimovic headed it home, although he appeared to be in an offside position, for his 26th league goal of the season.
Later on Saturday, French champions Montpellier, who still have an outside chance of qualifying for the Europa League, take on Valenciennes.
Sixth-placed Nice, who trail third-placed Olympique Lyon by five points, travel to Toulouse.