Saints comeback falls agonisingly short in thrilling clash with Harlequins
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Phil Dowson's side dragged themselves back from 35-15 down with 15 minutes to go, and they spent the final stages in the Quins half, only to be denied.
It was an incredible encounter, in which Saints had an avalanche of possession and territory.
They dominated the first half but somehow trailed 21-10 at the break, having proved porous in defence and profligate in attack.
Quins looked to have taken the game beyond Saints in the second period, but they refused to give in, storming back to six points down, bagging a try bonus point in the process.
But the losing bonus point was the only other one they were able to take as Quins kept their discipline superbly in defence, booting the ball out as they celebrated the win.
The game had started with Saints under huge pressure at their first scrum, conceding a penalty, which allowed Quins to kick to the corner.
And the home side took immediate advantage, rumbling over through hooker Jack Musk.
It was a horror start for Saints, who were 14-0 down after just eight minutes.
The away side should have had a try back when Rory Hutchinson and Alex Mitchell combined down the left, only for Mitchell to take too long and get tackled by Joe Marchant.
Saints were camped in the Quins half, but they were wasting chance after chance.
The away side won a penalty from a Quins put-in at the scrum, a good riposte after the early struggle, but Manny Iyogun knocked on when Saints worked a move to five metres out.
Penalty after penalty was coming the way of the black, green and gold, and eventually Dan Biggar decided enough was enough, taking a shot at goal, which he landed.
Quins struck back quickly though, making the most of some poor defending as Luke Northmore cruised in.
The hosts had barely been in Saints' half, yet they had scored three tries in 25 minutes, with Smith adding the extras to each.
It was already looking a long way back for Saints, but they continued to push on, dominating territory and possession.
And after Collins produced a fantastic 50:22, the away side finally had their first try, keeping hold of the ball before Lewis Ludlam finished strongly.
Biggar converted and the gap was cut to 11 points five minutes before the break.
Saints still had time to piece together another sensational move as they swarmed all over Quins, but some brilliant defending denied Hutchinson as he was bundled into touch.
The second half didn't start well for Saints as Lukhan Salakaia-Loto was yellow carded for shoulder contact to the head of Will Evans.
Quins kept applying the pressure and eventually Saints cracked, with Musk scoring his second from a lineout drive.
Smith converted and it really was a long way back for the black, green and gold, who were 28-10 down with 30 minutes remaining.
They kept fighting though and after staying patient close to the Quins line, Mitchell sent Tommy Freeman in out wide.
Biggar missed the conversion from the touchline as Quins raced out to put him under pressure.
Saints were starting to put things together again, but after they closed in on the Quins line, Alex Coles attempted to offload but lost the ball forward.
It was yet another moment in a game of nearly moments for Saints.
And it wasn't long until another one arrived as Ludlam was denied a try for what referee Tom Foley decided was a double movement.
The game seemed to be beyond Saints when Cadan Murley shrugged off three players to score in acrobatic fashion on the left.
Smith slotted the conversion from the touchline to take the lead to 20 points with just 15 minutes to go.
Saints reacted quickly as Dingwall sent George Furbank through a gap, with the full-back finishing well to give his side their third try with 11 minutes to go.
Biggar converted and Saints knew that one converted unanswered try for them would give them two points.
They were to get that try as Tom James forced his way over and Biggar converted, leaving Saints with four minutes to salvage a win.
Saints went through phase after phase in the Quins half, but the home side kept their discipline brilliantly to make sure of a dramatic success.
Harlequins: Green; Marchant, Northmore (David 60), Anyanwu, Murley; Smith, Care (Gjaltema 74); Marler (c) (Garcia Botta 65), Musk (Riley 60), Louw (Kerrod 58); Lamb, Herbst (Hammond 66); Kenningham, Evans, White (Jurevicius 66).
Saints: Furbank; Freeman, Dingwall, Hutchinson (Proctor 54), Collins (Sleightholme 54); Biggar, Mitchell (James 68); Iyogun, Matavesi (Haywood 66), Petch (Hill 58); Salakaia-Loto, Coles; Scott-Young (Moon 44), Ludlam (c) (Graham 66), Augustus.
Referee: Tom Foley