Following a totally one-sided first half Harlequins led 21-6. It should have been far more and a huge defeat for the Welsh seemed likely.
However, in a complete turnaround, Welsh played with great spirit in the second half to reduce the arrears to 24-19 and a remarkable comeback was on the cards before the visitors steadied the ship to run out narrow winners.
Harlequins rested England internationals Mike Brown and Ugo Monye from their matchday squad while the injured Gavin Henson was not ready to return for London Welsh.
Quins signalled their attacking intentions from the start as Ollie Lindsay-Hague fielded the kick-off deep inside his own 22 before setting off a sweeping movement which took the visitors into the home 22. George Lowe and Nick Evans both made vital contributions to the movement but an over-ambitious pass from Danny Care prevented Quins from opening the scoring.
The visitors continued to dominate but missed another chance when Evans was wide with a straightforward penalty attempt but with 15 minutes gone they took the lead with an individual try from Care. The scrum-half brushed aside some very weak Welsh tackling to run 15 metres for the score, which Evans converted.
The visitors threatened the Welsh defence almost at will and they soon extended their lead with a second try. Sam Smith did the spadework with a weaving run to create the score for Lowe, which Evans again converted.
Welsh were dealt another blow when lock Kirill Kulemin was shown a yellow card for a high tackle on Evans but they got on the scoreboard when Gordon Ross kicked a penalty after Quins were penalised at a scrum.
Kulemin returned but only in time to see his side concede a third try when Chris Robshaw barged over from close range. Evans converted but a second penalty from Ross made it 21-6 to the visitors at half-time.
Another careless pass from Care placed his side under pressure with Welsh wing Nick Scott being hauled down narrowly short. The hosts then had their best period of the match and should have scored but number eight Adam Balding chose to drive for the line instead of passing and was held up by the combined efforts of Evans and Tom Guest.
Welsh declined two kickable penalties in favour of more attacking options but when awarded two more, Ross made no mistake with both kicks for a 21-12 scoreline going into the final quarter.
The Quins were shaken but their nerves were calmed when Evans was successful with a penalty as Welsh committed another offence at a scrum.
Back came the hosts to collect their first try when Scott cleverly dummied over for the score, which Ross converted to reduce the gap to only five points.
With 13 minutes remaining Quins made the game safe with their bonus-point try, as Evans finished off some neat inter-passing with the outside-half converting for a match tally of 16 points.
Ed Jackson's interception try - converted by Ross - brought Welsh back within an unconverted try of Quins, but the visitors held on in a frenetic finish which also featured yellow cards for Quins lock George Robson and Welsh prop James Tideswell.
WASPS 29-15 BATH RUGBY
Christian Wade offered England head coach Stuart Lancaster a timely reminder of his talents with a superb solo try in Wasps' 29-15 victory over Bath.
The 21-year-old wing danced his way past three Bath defenders on the stroke of half-time to turn the game in the Adams Park outfit's favour - taking them into the Aviva Premiership play-off places in the process.
Wade's sixth Premiership try of the season handed Wasps their sixth win in eight games. Bath had a first-minute try from Nick Abendanon and Michael Claassens' effort shortly afterwards, but Wade's trickery and Stephen Jones' majestic right boot - which gathered 17 points in total - proved too much with Chris Bell's late effort the icing on the cake.
Bath got off to the best possible start with a try after 30 seconds for Abendanon in the corner. The full-back ran down the right touchline, evading two challenges, after catching Jones' high kick on the halfway line. Stephen Donald missed the tough conversion and moments later the visitors were penalised for offside and Jones added three points.
The game erupted when Tom Varndell was sin-binned for an alleged spear tackle on Horacio Agulla. And, after Jones missed a penalty, Bath capitalised on the one-man advantage and Wasps fell further behind on 15 minutes when Claassens bundled over in the corner. Donald converted from the touchline for a 12-3 lead.
Jones, returning to the Wasps starting line-up, struck an easy penalty on 21 minutes and shortly after he notched another to bring the hosts within three points of Bath. Dom Day was then sin-binned for bringing down James Cannon in the air but it was Bath who had the next scoring opportunity. Donald, though, missed his penalty.
Kyle Eastmond nearly forced his way in but Elliot Daly cleared to touch just 20 yards from the line. Daly then missed a penalty before Wade's moment of magic, beating three challenges on his way into the corner after Jones' long pass. Jones converted from the touchline on the Bath 22 for a 16-12 half-time lead.
Within two minutes of the restart Bath had reduced the arrears to one point, with Tom Heathcote, who had replaced Donald at the break, slotting a penalty. Wasps responded well though, a prolonged attack ending with Billy Vunipola driving his way towards the line only for his effort to be ruled out by the TMO.
The dynamic number eight failed to ground the ball but his disappointment was eased on 54 minutes when Jones added his fourth penalty of the game. Francois Louw then snaffled the ball well only for team-mate Dave Wilson to be penalised, with Jones making them pay with another three points to take Wasps seven points clear at 22-15.
Replacement Nicky Robinson missed the chance to put breathing space between the sides with a 69th-minute penalty but Bell's late try, after a neat offload from Joe Simpson, sealed the victory, with Abendanon's sin-binning for a trip ending his afternoon on a sour note.
SARACENS 32-12 SALE SHARKS
Owen Farrell showed perfect timing to demonstrate his worth to Stuart Lancaster ahead of the England coach naming his Six Nations squad on Wednesday as the fly-half kicked 22 points as Saracens eased to a 32-12 victory against bottom club Sale.
In the last-ever Aviva Premiership match to be played at Vicarage Road, Farrell hammered over eight kicks from eight attempts while David Strettle and Charlie Hodgson collected second-half tries to help Saracens to a comfortable win.
The defeat leaves Sale five points behind London Irish at the foot of the table and they only had four Danny Cipriani penalties to show for their efforts.
Their defence on the whole was solid, but the same could not be said of their discipline and their attacks were easily blunted by the hosts.
However, they started the game on the front foot and led within five minutes thanks to a Cipriani penalty.
Just after the restart, the fly-half got lucky when his kick from inside the 22 was charged down by George Kruis only for the ball to bounce over the dead ball line.
But Cipriani soon doubled the Sharks' lead when he belted over a terrific penalty from just inside his own half, although Saracens were to halve the deficit through Farrell.
Five minutes later, Farrell brought the score level with another penalty, left-of-centre on the 22-metre line, after James Gaskell was penalised for not rolling away.
It was all Saracens at that point and they soon led for the first time with Farrell again deadly from the kicking tee, making a shot from 40 metres.
The England man then repeated the trick just a couple of minutes later to extend the gap to six points, with Sale's discipline letting them down badly.
They were to gift Farrell another shot at goal in the 28th minute, one which he did not pass up and despite not threatening the Sale tryline, Saracens found themselves 15-6 ahead.
It was to remain that way until half-time with both sides struggling to get out of the middle third of the pitch.
Eleven minutes into the second half, Cipriani brought his team back within six points with a penalty made all the more kickable thanks to some Saracens backchat.
But within two minutes, those points were rendered meaningless with Farrell again making it look easy with his sixth penalty, taken from just inside the Sale half.
A rare moment of attacking promise for either side just before the hour-mark then came to nothing when Sale's Kearnan Myall failed to find any support to his left just outside the Saracens 22.
But Cipriani was to boot the visitors back within touching distance with an excellent, and somewhat fortuitous, strike from the halfway line which hit both posts before dropping over.
However, Saracens took a huge step towards victory in the 63rd minute - just after bringing on former Shark Hodgson.
A strong scrum led to Richard Wigglesworth passing to Hodgson, who with his first touch gave a quick pass to allow Strettle to sprint in under the posts, giving Farrell an easy conversion.
The result was made safe with 10 minutes left and it was roles reversed from the first try as Strettle's deft grubber was simple for Hodgson to pick up and score.
Farrell kicked the extras again and the gap was up to 20 points, meaning that Sale were going to go back up to Salford with nothing to show for their efforts.