The capital club extended their advantage at the summit, though, were made to work by the Corsicans, who managed to end Sirigu's 948-minute record without conceding a league goal
Jeremy Menez opened the scoring with a finely-taken goal after a frustrating first half which ended goalless, before Zlatan Ibrahimovic emerged from the bench to score a penalty. Another substitute, Wahib Khazri scored an audacious free-kick to ensure a grandstand finish before Ezequiel Lavezzi ended any doubt late on.
With the league's best attackers facing the side with the worst defensive record, it was unsurprising to see the hosts dominate the early opening exchanges, as Gameiro set up Lucas, though neither could test France international Mickael Landreau in the Bastia goal.
Whereas the Corsican side aimed to better their local rivals Ajaccio, who managed a 0-0 draw in the capital last month, they should've been one down after failing to even clear their lines, as the duo combined once more - Gameiro's cutback being fired uncharacteristically wide by the Brazil international.
The visitors where nonetheless posing a dangerous threat at the other end, with Lille-bound Florian Thauvin, subject of a January bid from Newcastle, teasing Salvatore Sirigu with a shot from an acute angle, before Sylvain Armand was forced to hurriedly clear a threatening free-kick.
Nevertheless, the league leaders were proving too much for Bastia's rearguard as a golden chance went begging. Jeremy Menez embarked on a mazy run past a gaggle of defenders before dragging his shot wide, though not before Gameiro ghosted in at the back to hit the post in a frustrating first half.
Despite being booed off at the break, the Parisian outfit once again started the second period in much the same way. Maxwell took advantage of a lucky break and some lax defending to thread in a dangerous ball which was hurriedly dealt with by Sylvain Marchal.
With the frustration and noise building inside the Parc, the hosts were once more denied by the brilliant Landreau, who turned away Gameiro's audacious back-heel off the line after getting away from Fethi Harek. However, the resistance was then broken.
Pastore raced into a decent position before teeing up Menez, who after taking a few touches, proceeded to cut in on his left and fire an unstoppable shot past Landreau - punishment for Bastia, who began to noticeably fade after the break.
The introduction of Ibrahimovic added another dimension to the hosts, who went further ahead after Julian Palmieri felled Lucas in the area. Up stepped the Swede to tuck away his 21st league goal of the season, to equal the total set last year by Olivier Giroud and Nene, though with 14 games still left to play.
Menez could've added a spectacular third after once more getting the better of three defenders, before being denied by an equally impressive Landreau. But Bastia hit back immediately.
Wahib Khazri came off the bench to launch a stunning free-kick past Sirigu, who was off his line - a controversial way to see his club record of more than 940 minutes gone without conceding a league goal go up in smoke. The substitute almost grabbed a second, as Armand hurriedly cleared away a potential equaliser.
Lavezzi turned in a third, following a fluent attacking move, to confirm a victory which moves the pressure back onto the chasing pack of Olympique Lyonnais and Olympique de Marseille.
Following the result, Ancelotti's men travel to the Mestalla for their Champions League first knockout round first leg on Tuesday, as Bastia turn their attention to a tantalising home fixture against high-flyers Nice next weekend.