Pellegrini glad City's fate in own hands
Yaya Toure's third-minute penalty gave City the lead at Eastlands before Rickie Lambert equalised from the spot for Southampton.
But two goals in first-half stoppage-time from Samir Nasri and Edin Dzeko turned the match City's way before substitute Stevan Jovetic made it 4-1 in the 81st minute.
Victory left City just a point behind Liverpool, who play West Ham on Sunday and two behind current leaders Chelsea, with Pellegrini's men having two games in hand on both of their title rivals come the end of this weekend.
Now City travel to face Liverpool next weekend knowing that if they win all their remaining seven matches, they will be champions.
"We just depend on ourselves until the end of the season -- it's important to continue playing the way we did in the second half because I thought it was a great performance," said Pellegrini.
"We are enjoying a very good second round of fixtures where the results are much better than the first half," the Chilean added.
"We've had four wins and one draw in our last five games -- maybe we had low performances in the month of February and at the start of March but I think that now the team has come back to its best performances."
- 'Rodriguez not looking good' -
For Southampton the pain of defeat was made worse by the sight of Jay Rodriguez being carried off on a stretcher in the first half after he twisted his right knee having landed heavily as he jumped for the ball.
And while the full extent of his injury is yet to be discovered, the England forward could now miss out on a place in Roy Hodgson's squad for this June's World Cup finals in Brazil.
"To be completely honest it is not looking good," said Southampton manager Mauricio Pochettino.
"It's not going to be good for him, it's not going to be good for the team.
"I don't really know what's happening with him or when we are going to see him again in a Southampton shirt.
"All the players are really affected, not just by the result, but more than anything by the doubt of how severe his injury is."
Rodriguez has been included in the last two England squads, collecting one cap in the friendly against Chile but a recent run of five goals in four games had strengthened his case for a World Cup place.
The striker's injury capped a frustrating day for Southampton, with the south coast club convinced they were harshly treated by the match officials.
Pochettino claimed referee Chris Foy was wrong to award the penalty with which City opened the scoring and he was angry at the failure of the officials to spot a clear offside in the build-up to Nasri's goal.
"I don't think you can give a penalty like that after two minutes," he said.
"The second was clearly offside, two metres offside, and it felt like a game you play at school where you get these dumb refereeing decisions.
"There are two decisions that killed the game. The penalty which we conceded and the second goal, they killed the game.
"The refereeing decisions have to be fair and they were not fair today. It can influence any player, when you get a bad call, and it influenced our players today."