Blackburn Rovers substitute Lewis Mansell scored the only goal as his side beat Watford 1-0 at Wealdstone to go top of Group B.
Mansell fired into the corner of the net via the far post in the 73rd minute to seal Rovers' fourth successive league and cup win.
Rovers have seven points, one ahead of Southampton, while Cardiff City have five and Watford four.
Each side have two matches remaining. The top two progress to the knockout stages.
See: Watford report
Next fixtures
Watford: 26 Feb v Cardiff (H) PL Cup
Blackburn: 16 Feb v Brighton (H) PL2
