LONDON Welsh upset the formbook in devastating style to inflict a potentially decisive defeat on Bristol at the Kassam Stadium last night.
The Exiles would have taken any sort of victory, but roared back with real style after a slow start.
Bristol failed to build on their lead and Welsh were good value for their three-try win.
They will take a 19-point lead to the Memorial Stadium next week and, if Welsh keep their heads, it will require something spectacular from Bristol to turn the tie around.
Bristol struck the first blow on five minutes after fly half Nicky Robinson went for the corner from a penalty and flanker Marco Mama was driven over for a well executed try.
The visitors looked strong and Welsh, who lost full back Seb Stegmann to injury early on, seemed nervy.
But the tide turned with two quick tries around the quarter mark.
Firstly, prop Nathan Trevett was driven over in a line-out move every bit as impressive as Bristol's earlier.
Fly half Gordon Ross converted this and a second score from wing Nick Scott soon afterwards.
That was a spectacular breakaway effort, with Scott running in from halfway after taking a quick pass from hooker Nathan Morris.
Welsh grew visibly in confidence and started to control the play.
Bristol suffered a major blow when captain and former Wales international Ryan Jones was stretchered off.
To add to their woes, scrum half Luke Baldwin had to be helped off with another injury not long after.
And a great half for the hosts ended on a high note with Ross slotting a penalty for 17-5.
The second period began in even better fashion, with Welsh quickly gaining territory and kicking for the corner.
Their driving maul was again far too good and flanker Carl Kirwan applied the finishing touch for try No 3 converted by Ross.
Bristol threatened in patches, but Welsh almost conjured a fourth try on the left - Scott just unable to hold the scoring pass.
His side were soon on the attack again and only some illegal defence from Bristol flanker Nick Koster prevented another score.
Koster was sin-binned and Ross gleefully slotted the penalty for 27-5.
Bristol were now desperate for anything, any having eschewed kickable penalties all season, cut their losses with replacement fly half Adrian Jarvis landing a simple three-pointer.
That proved it for the scoring and could not dampen a superb night for the Exiles, who must now be favourites for promotion.
London Welsh: Stegmann (Awcock 13), Crane, May (capt), Jewell, Scott, Ross, Cook, Trevett (Bristow 49), Morris (Vella 66), Edwards (Tideswell 49), Spencer, Corker, P Browne (West 62), Kirwan, D Browne (Thorpe 50). Rep not used: R Lewis.
Bristol: Wallace, Watkins (Hughes 67), Tovey, Mosses, Short, Robinson (Jarvis 59), Baldwin (Tipuna 38), Traynor (Hobson 78), Johnston (Lawrence 49), Hobson (Hall 49), Glynn (Skirving 73), Townson, Koster, Mama, Jones (capt, Eadie 30).
Referee: W Barnes (Gloucestershire).