After a slow start in which the Blues went behind, two goals from debutant Demba Ba plus strikes from Victor Moses, Branislav Ivanovic and Frank Lampard maintained our unbeaten run in the FA Cup third round which stretches back to 1999.
It was Ba with a close-range finish who levelled the scores, with Moses putting the visitors ahead on the stroke of half-time. The second half was one-sided although Ross Turnbull pulled off a smart double save.
Ivanovic headed his goal soon after the restart and after the impressive Ba had stretched the lead, the Senegalese striker was denied a hat-trick by a good save.
Substitute Lampard from the penalty spot went level with Kerry Dixon as Chelsea’s second highest goalscorer, and at the same time joined Bobby Tambling as the club’s highest FA Cup scorer. He now has 25 goals in the competition.
It was instantly clear from the team selection that Rafael Benitiz is taking the FA Cup seriously. Back in came Juan Mata, Eden Hazard, Ramires and Ashley Cole who was captain.
Demba Ba made his debut and David Luiz continued in midfield, his compatriot Ramires the one alongside him this time.
Southampton lined up with former Chelsea junior Jack Cork at right-back. Striker Rickie Lambert started on the bench so Jay Rodriguez played up front, with James Ward-Prowse coming in to their midfield for injured Ramirez.
Despite having come up to the Premier League, this Southampton team were still new opposition for Chelsea but historical omens warned against any complacency with the Saints having progressed the four previous occasions we were drawn against each other in the FA Cup.
It took a long time for Chelsea to get into our stride in this third-round tie however. On six minutes Mata won a corner, Southampton cleared their area a couple of times but on the second occasion when the ball fell to David Luiz outside the box, the Brazilian sent his effort up into the dark grey sky. Moses waltzed past Cork in an attack soon after but his delivery came to nothing.
Chelsea had to defend a 14th-minute corner, headed away strongly by Ba at the near post, but the new arrival had been given little chance to show what he could do at the other end. The first 20 minutes had been nothing to write home about.
On 21 minutes the home side took the lead. There was perhaps not enough pressure on Jason Puncheon in possession but the right-midfielder played a very good diagonal pass and the Blues were in trouble. Jay Rodriguez was in behind Cahill and although Turnbull closed fast, the no. 9 kept his head to score.
Neither keeper had previously been tested but now Chelsea knew we had to improve very quickly.
Hazard drilled a curling shot only just wide on 25 minutes having been supplied by a Ramires pass, and soon after Artur Boruc in the Saints goal did well to get a fist to a Mata corner with Cahill not far away from making contact.
There was a flash of quality from Ba on half an hour when he dribbled out from a tight spot towards goal, but was dispossessed for a corner. On 34 minutes he was in the right place at the right time to put his new side level.
Hazard, who had played mostly on the right, had briefly come over to the left and was found by a precise flick by Moses. The Belgian attacked and picked out Mata in space in the area, and showing technique to drool over, our joint-topscorer in the FA Cup last season, chipped the keeper. There was every chance a defender could have cleared the ball off the line but Ba had the burst to get his leg to it inches out and make sure the ball went in.
Now Chelsea went in search of a half-time lead and Boruc in the Saints goal was shaky in dealing with shots from Moses and Azpilicueta.
Hazard was looking capable of causing problems every time he attacked down our left, and it was from that flank our second goal originated seconds before the half-time whistle.
Cole played a precise square ball into the area to Moses and from 15 yards out, the Nigerian gave Boruc no chance as he powered a shot beyond him.
A Mata corner very nearly fell to Ba in front of goal five minutes after the restart but we only had to wait one more minute to go further ahead. Mata was involved again, bringing the ball in from the left and placing his cross on to the head of Ivanovic for the Serbian to head down and in from six yards out.
The fourth goal, scored on the hour, was a model in patience and good movement. Moses and Ramires switched the play in front of the penalty area with the ball played out to Hazard, this time in space on the right. He hit a low diagonal ball into the box and Ba read it well, scoring with a sharp near-post finish.
Frank Lampard replaced Ivanovic on 64 minutes, with David Luiz dropping into defence. Marin came on for Moses eight minutes later but between those changes it took good goalkeeping at both ends to prevent goals.
Turnbull’s was a fine double stop, blocking shots from Davis and De Ridder. Then Moses crossed to the far post and Ba rose high to head down, but Boruc scrambled the ball out from on the line.
Inside the final 10 minutes, Lampard sprayed a superb ball out to Marin in space and the cross from the German was handled. It was a penalty. From the spot Lampard smashed home his milestone goal.
The two headline writers almost combined for a goal in stoppage time. Ba charged towards goal on the right and picked out Lampard on the penalty spot with a pass but the ball slipped from the midfielder’s control.
We find out our fourth-round opponents when the draw is made tomorrow (Sunday) at around 3.30pm.
Chelsea (4-2-3-1): Turnbull; Azpilicueta, Ivanovic (Lampard 64), Cahill, Cole (c); Ramires, David Luiz, (c); Hazard, Mata (Oscar 78), Moses (Marin 72); Ba.
Unused subs Hilario, Ferreira, Bertrand, Torres.
Scorers Ba 34, 60, Moses 46, Ivanovic 51, Lampard 82 pen.
Southampton (4-2-3-1): Boruc; Cork, Fonte (c ) (Hooiveld 63), Yoshida, Shaw; Schneiderlin (De Ridder 54), S Davis; Puncheon (Lee 77), Ward-Prowse, Do Prado; Rodriguez.
Unused subs K Davis, Reeves, Mayuka, Lambert.
Scorer Rodriguez 21.
Booked De Ridder 77, Ward-Prowse 88.
Referee Mike Dean.