However, following the Blues kick off, the home team were penalised early on and Jack Cuthbert slotted it between the posts to give Bath the early lead 0 - 3.
The Blues struck back within a couple of minutes with good build-up play by Tom James laying the platform for Nicky Robinson to flip the pass inside to his brother Jamie. Jamie in turn did well to shake off a tackle and Molitika had the strength and the speed to touch down under the posts. Blair converted.
The Blues built a further early lead when they were awarded a penalty from a scrum and Blair converted from 40 yards out.
However, the Blues were putting themselves under pressure with handling errors including one incident seeing the ball fly backwards into their own 22 requiring the home teams defensive line to scramble back after it. However, the Blues did manage to extend their lead further with another Ben Blair long range penalty, making the score 13-3.
Bath continued to put Cardiff Blues under pressure as the English visitors dominated both territory and possession. Scrum half Scott Beman nearly slipped through for a try after some good build up work by the forwards. Bath failed to get points on the board when Jack Cuthbert kicked a penalty wide and the Blues managed to clear their lines. It remained a dangerous time for the home team, but they managed to hang on with some determined defence to go in at the break with the half time score Cardiff Blues 13 Bath Rugby 3.
Half Time Cardiff Blues 13 Bath Rugby 3
The Blues started the second half with a bit more purpose and direction to their game and were playing most of the rugby in Baths half and looking dangerous.
The Blues were dealt a blow when Shanklin was shown a yellow card and Maddock converted the penalty. From the restart Bath were penalised and Blair notched a penalty back for the Blues.
Crocket went on a superb 60-yard run that took Bath back into the Blues twenty-two and they were again under pressure before Blair cleared well. However, Bath counter attacked with a superb run by Higgins who got within a yard of the line before Spice high tackled him and Bath gained a penalty try.
Mustoe did superbly to charge down the conversion in front of the posts though.
Again from the restart Bath were penalised giving the Blues the chance to extend their lead once more, but Blair struck it just wide this time.
The Blues however looked to have scored when Jamie Robinson broke through the defence and the ball was knocked back over the tryline by Bath for Spice to touch down. The try wasnt awarded due to an incorrect knock on decision, but the Blues did well to be awarded a scrum. Molitika picked up from the base of the scrum and drove forwards and Filise was at hand in the ruck to crash over against his old team.
Bath son struck back when Faamatuainu made a powerful break through the midfield and good hands saw Andrew Higgins touch down with Berne converting, taking the English league leaders back within 3 points.
With the seconds ticking down the Blues had one foot firmly in the semi finals but with only a three point cushion it was never a certainty. The tense finish was calmed with Blair kicking the final points with a penalty, to secure a 24-18 win and a second semi final in three years for Cardiff Blues.
Semi finals to be played at Ricoh Arena, Coventry on Saturday 28th March. Draw to be made on Sunday 2nd November live on BBC at 5.45pm.
Cardiff Blues 15 Ben Blair 14 Richard Mustoe 13 Jamie Robinson 12 Tom Shanklin 11 Tom James 10 Nick Robinson 9 Jason Spice (72off) 8 Mark Lewis (62off) 7 Robin Sowden Taylor 6 Maama Molitika 5 Paul Tito (c) (62off) 4 Deiniol Jones 3 Taufaao Filise 2 Gareth Williams 1 John Yapp (32off) 16 Gary Powell (32on) 17 Ross Johnson 18 Scott Morgan (62on) 19 Martyn Williams (62on) 20 Richie Rees (72on) 21 Ceri Sweeney 22 Gareth Thomas
Bath Rugby 15 Jack Cuthbert 14 Joe Maddock 13 Alex Crockett (c) 12 Shontayne Hape 11 Andrew Higgins 10 Shaun Berne 9 Scott Bemand 8 Daniel Browne 7 James Scaysbrook 6 Andy Beattie 5 Danny Grewcock 4 Peter Short 3 Duncan Bell 2 Pieter Dixon 1 David Flatman 16 Rob Hawkins 17 Aaron Jarvis 18 Nathan Catt 19 Stuart Hooper 20 Jonny Faamatuainu 21 Mike Baxter 22 Tom Cheeseman
Tries - Molitika x 1 Filise x 1
Penalties Blair x 2
Cons - Blair x 2
Tries penalty try Higgines x1
Penalties Jack Cuthbert x 1 Maddock x 1
Cons Berne x 1
Yellow card Tom Sahnklin.