But the visitors responded before the half hour, James Ward-Prowse making no mistake from the spot after Jannik Vestergaard was hauled to the ground from a corner.
Pep Guardiola’s men felt they should have had a penalty themselves moments later when Foden lost his footing at the crucial moment after capitalising on a McCarthy error, but referee Jon Moss waved away the claims.
Undeterred, we were back in front with half-time approaching when Mahrez cut in on his left and whipped a delightful effort into the bottom corner from the edge of the box after intercepting Che Adams’ pass across his own goal.
And the Algerian was in the thick of the action once again just before the break, with his curling effort cannoning back off the post and into the path of Gundogan, who volleyed home from inside the six yard box.
Mahrez then grabbed his second and City’s fourth early in the second half, dancing his way through a number of Southampton challenges inside the box before firing into the net.
Our defiant visitors found an immediate response, when Stuart Armstrong’s effort on goal landed fortuitously into the path of Che Adams, who cooly found the back of the net to give Southampton hope of an unlikely comeback.
It was an action-packed evening at the Etihad - check out the gallery below to see how it unfolded…