Sheffield United showed why they have seized control of the race for promotion from League One with a devastating display of firepower at Roots Hall.
Southend, previously unbeaten in 10 outings and harbouring promotion ambitions themselves, entered this rearranged fixture with high-hopes of causing an upset but were swept aside by goals from Ethan Ebanks-Landell, Jack O'Connell, Caolan Lavery and Kieron Freeman.
The result, which saw Chris Wilder's side move four points clear of second-place, means United have now won all of their previous six games. But an injury to new signing Samir Carruthers, who limped-off soon after making his debut, will have removed some of the gloss.
Wilder felt confident enough to predict earlier this week that Ebanks-Landell will be spending the rest of the season at Bramall Lane following talks with his parent club Wolverhampton Wanderers. Another solid display at the heart of United's backline demonstrated why the 49-year-old is so keen to retain his services. But the on-loan defender also made his presence felt at the other end of the pitch, scoring for the fifth time this season before Cox dragged Southend level.
Phil Brown's side, who started the match in seventh place, had won comfortably when the two clubs met at Bramall Lane five months ago. They impressed again - Cox levelling the scores - before discovering United are now a very different beast to the one beaten 3-0 on home soil in August.
O'Connell restored their lead just before the break and, when Lavery pounced in the second-half, the match was effectively over. Freeman twisted the knife when he netted for the third time in as many games during the closing stages. A remarkable return from United's wing-back and probably man-of-the-match too. Substitute Jermaine McGlashan grabbed a late consolation on the stroke normal time. United were deserved winners but Brown's charges showed enough to prove they can secure a top six place.
Two goals inside the first 18 minutes underlined the frenetic pace of the game. Although they were shared, it was a situation which suited Southend far better than Wilder's side given that the visitors could, probably should, have been out of sight by the time Cox cancelled-out Ebanks-Landell's early effort.
Billy Sharp had already gone close by the time the on-loan centre-half, who has scored more goals than any other defender in the competition so far this term, met Mark Duffy's cross and sent a glancing third minute header across Ted Smith and into the far corner of the net. Kieron Freeman failed to hit the target from close range after dancing through Southend's rearguard as United threatened to run riot. But Wordsworth's strike, which sneaked through a crowd of players from the edge of the box, breathed new life into Brown's players who, up until that moment, looked close to being counted out.
Simon Moore has developed a lovely habit of making key saves at crucial moments. The one which prevented Anthony Wordsworth from handing Southend the lead midway through the opening period was definitely one of those.
United, recalling Paul Coutts and Duffy after they started Monday's contest at Bury on the bench, became increasingly ragged as their grip on the game weakened. Neverthless, a stray back pass from Jason Demetriou nearly provided Sharp with a chance seven minutes before the interval. Smith, aware of the danger, dashed off his line to clear from United's captain.
But it was a reminder that the fixture remained delicately poised.
Wilder had just wheeled away in frustration on the touchline when O'Connell restored United's lead with a well-taken 42nd minute finish. Anton Ferdinand's foul on Caolan Lavery presented with Duffy with another opportunity to test Southend's aerial prowess. Much to Brown's disgust, they failed again by allowing the ball to reach the onrushing O'Connell who turned it home.
Predictably, Southend pressed at the start of the second-half but struggled to carve a clear cut opening. United ensured that lack of creativity was punished in the most ruthless fashion when Lavery extended their advantage in the 72nd minute before providing an assist for Freeman four minutes later. McGlashan beat Moore after beating Freeman and Ebanks-Landell before Cox hit the woodwork as Southend tried, but ultimately failed, to set up a dramatic finale.
Meanwhile, United's under-18's beat Birmingham City 3-1 at Shirecliffe this morning. Joseph Cummings, Tyler Smith and David Parkhouse scored for the hosts.
Southend United: Smith, Demetriou, Thompson, Ferdinand, Timlin, Atkinson (Hones 72), Leonard, Wordsworth, Fortune (McGlashan 60), Cox, Ranger. Not used: White, O'Neill, Bridge, Kyprianou, Bexon.
Sheffield United: Moore, Fleck, O'Connell, Basham, Lavery, Sharp, Coutts, Freeman, Ebanks-Landell, Duffy (Carruthers 68) (Scougall 82), Lafferty (Done 56). Not used: McNulty, Ramsdale, Wright, Clarke.
Referee: Andy Davies (Hampshire).