Kenny Miller's brace ensured Cardiff inflicted only a second defeat of the season on early npower Championship pacesetters Southampton as they triumphed 2-1.
The Scot's well-taken double, within seven second-half minutes, settled a finely-balanced affair which provided little in the way of clear-cut openings in the first half.
That all changed after the break and Miller broke the deadlock with a brave header past onrushing Saints keeper Kelvin Davis from David Marshall's long ball upfield, before then finishing off good work from Aron Gunnarsson.
Substitute Steve De Ridder did grab an injury-time consolation for Saints, but it is now two games without victory for Nigel Adkins' side after they salvaged a late draw at Burnley on Saturday, although Saints remain top of the table on goal difference.
Sixth-placed Cardiff, meanwhile, have still only suffered one defeat all season and are now unbeaten in six, although four of those have been drawn.
Middlesbrough missed the opportunity to move top but remain unbeaten in the league following a goalless bore draw with Leicester.
Having struggled to adapt to life back in the second tier over the last two seasons, Boro have enjoyed an excellent start to this campaign and knew victory over the Foxes would see them move to the summit if Southampton slipped up at Cardiff.
But, despite Saints being beaten 2-1, Tony Mowbray's side remain second on goal difference after a second home goalless draw in five days.
On a day when the headlines were dominated by Manchester City striker Carlos Tevez's refusal to go on as substitute during Tuesday night's 2-0 defeat to Bayern Munich, his City team-mate Michael Johnson - on loan at Leicester - made his first league start for three years.
The England Under-21s international was featuring in City's first team in the not too distant past, but serious injuries have limited him to infrequent appearances over the last 36 months.
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