Celtic moved 14 points clear at the top of the Scottish Premiership as Scott Sinclair's first-half strike was enough to secure a 12th straight league win.
Celtic, showing eight changes from their weekend win over Dundee, edged in front after only 16 minutes when Sinclair's clipped finish exposed a static defence after a free-kick.
Both sides had chances to add to the scoring, but Celtic secured three points despite living dangerously.
An 18th consecutive win in Glasgow derbies against Thistle takes Brendan Rodgers' side further clear of city rivals Rangers at the top of the Premiership - and they have another game in hand to come.
Thistle, who had the chance to jump to eighth place, were unable to use their extra game over their rivals at the other end of the table and remain bottom having now gone seven league games without a win.
The defending champions did not deliver the best of themselves against dogged opponents, but they found the means to win the game.
That in itself is a handy attribute and Brendan Rodgers will have been cheered by Sinclair's goalscoring return.
Despite being out of sorts at times, Celtic still had opportunities to secure a more convincing win in the second half, but they found Tomas Cerny a difficult obstacle to beat in the Thistle goal.
Moussa Dembele saw an early effort after the break deflected wide, then the Thistle goalkeeper saved well from Patrick Roberts.
The attention was soon drawn to 18-year-old Calvin Miller making his competitive debut at left-back, but he wasn't alone in the side as a youth development product since Callum McGregor and Liam Henderson also both started the match.
Miller's first involvement was to win a tackle and he mostly looked composed on the ball and eager to surge upfield.
He might have scored, too, when a series of one-twos around the edge of the area ended with him cutting into the box, but his effort was smothered by Cerny.