At a packed BT Sport Cardiff Arms Park the Blues claimed a second derby success on the spin over their Welsh rivals despite gifting first-half scores to Ken Owens and DTH van der Merwe.
Scarlets however remain top of the table thanks to their losing bonus point.
Phil Burleigh's late try helped Edinburgh complete the double over defending champions Glasgow Warriors as they clinched a 14-11 win at BT Murrayfield.
Burleigh's crucial score came six minutes from time but the visitors had to see off an incredible 26 phases at the death to secure victory.
Dan Evans touched down against his former club as Ospreys' first-half heroics were enough to deny Newport Gwent Dragons their first away win of the season as they claimed a 12-7 victory at the Liberty Stadium.
Tries from Evans and Hanno Dirksen had the hosts 12-0 ahead at half-time but the lead could have been bigger had Sam Davies been more accurate with his kicking.
Ed Jackson powered over for the Dragons and Jason Tovey converted with 15 minutes remaining but Ospreys held on for their fifth consecutive GUINNESS PRO12 victory.
Leinster made it six wins in a row by overcoming miserable conditions to shut out Connacht 13-0 and leapfrog them into second place in the table.
The visitors, who were handed their third consecutive defeat, found themselves within range at the interval but Josh van der Flier bowled over shortly after the break while Sexton added the conversion before Ian Madigan put the icing on a crucial win with a late penalty.
Luke Burgess scored his first GUINNESS PRO12 try as Zebre beat Benetton Treviso 18-8 to win three games in a row for the first time in their history.
Munster got back to winning ways with a narrow 9-7 victory over Ulster at Kingspan Stadium.
The visitors came into the match off the back of three consecutive defeats but two penalties and a drop goal from Ian Keatley got them over the line.
A converted Louis Ludik try put Ulster in control early on but Keatley's boot soon turned things around, while Paddy Jackson was unable to claw something back as he missed from the tee with two minutes left.
TRY OF THE WEEKEND
Giulio Toniolatti picked up his first GUINNESS PRO12 try of the season to quash any hopes of a Treviso comeback.
With the hosts' energy dwindling, Zebre burst through the middle before Matteo Pratichetti was played in.
With a quick look over his shoulder he funnelled the ball over to Toniolatti who shrugged off a tackle to score right in the corner.
BREAK OF THE WEEKEND
Dan Evans came back to haunt his former club Newport Gwent Dragons to put Ospreys ahead in the first half of their Welsh derby.
Sam Davies may not have impressed with his boot on the day but he deftly flicked the ball to Evans who had already built up a head of steam.
The Swansea man stormed into the space ahead of him, skipping away from two last-ditch tackles to touch down.
PLAY OF THE WEEKEND
Dan Fish found himself on the end of a dogged Cardiff Blues move to reduce their deficit against Scarlets, with man of the match Tom James providing the assist.
A swift four-pass move stretched Scarlets' defence and gave James space to charge forward and draw attention before playing in Fish, who flew over.
PLAYER OF THE WEEKEND
Ian Keatley scored all nine of Munster's points during their tight win over Ulster, his accurate kicking ending a spell of three consecutive defeats for his side.
Two penalties and a drop goal earned Keatley man of the match honours but the Dublin native was in the thick of it from the start.
As well as keeping perfect form from the tee, Keatley kicked well from hand to keep the pressure on the hosts.
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