A last-gasp try by Chris Ashton meant that London Irish failed to take advantage of Saracens' slip, as Northampton downed the Exiles 24-22.
With England coaches Brian Smith and John Wells watching, both teams included strong contenders for manager Martin Johnson's Six Nations elite player squad, which will be announced on January 13.
Ashton, Ben Foden, Dylan Hartley and Courtney Lawes of Saints, and visiting players Delon and Steffon Armitage, Paul Hodgson, Davis Paice and Nick Kennedy all had extra incentive as second in the Premiership took on third.
Saints came under immediate pressure, and spent most of the opening five minutes showing their defensive mettle.
But once the hosts broke out, Irish were caught offside, and fly-half Stephen Myler opened the scoring with an eighth-minute penalty.
Irish equalised in the 12th minute when Saints offended at a ruck. Chris Malone's penalty from close range had to be one of the easiest asked of him this season.
The capacity crowd were disappointed by the amount of kicking employed by both teams during the opening quarter, but this was a tight battle between teams very difficult to separate.
Referee Sean Davey penalised Saints for what seemed a harsh offside on 24 minutes, and Malone put Irish ahead with a fine kick from wide on the right.
Before Northampton could strike back, Malone scored his third penalty in the 27th minute, this time awarded for not releasing. Davey looked ready to produce the game's first yellow card after Steffon Armitage was penalised for ball-killing on the Irish line, but the official chose instead to lecture the flanker in the presence of captain Bob Casey before awarding Saints a penalty, duly hammered home by Myler eight minutes from half-time.
But Irish ended the half as they began it, battering at Northampton's defence without producing the cutting edge to break it down. Their best efforts were two failed drop-goal attempts by Malone.
It was Northampton who took the initiative after the restart, forcing Irish back and into conceding a penalty on the left wing. Myler kicked for touch, Dylan Hartley's lineout throw set up a drive and Phil Dowson was shunted over the line in the 43rd minute. Myler's conversion attempt flew wide.
The way Saints were forcing the tempo it was clear they had received some sharp words of advice from their management during the interval, and it was fully 10 minutes before Irish could defuse the offensive when awarded a penalty for not releasing.
Irish tried to launch their first attack of the half, but the ball was pinched off the ground and it took a suspiciously late tackle by John Rudd to prevent Ashton finding a route to the goal-line.
Intense pressure told in the 55th minute when Irish were pinged for pulling down a maul. Saints again took the lineout option on the left and drove for the line which earned another penalty for offside. With referee Davey close to penalty try time, it was Irish who stunned the crowd by scoring their second try in the 58th minute.
Saints were attacking towards their right wing, but a pass meant for Ashton was intercepted by Peter Hewat who found the legs to race from just outside his own 22 to score. Malone added the conversion.
As if snapped out of a daze, Irish changed gear and increased their lead inside five minutes. Northampton were trapped offside and Malone made no mistake from in front of the posts.
Foden put pressure on Irish with a clever chip-kick that forced Hodgson to dive on the ball - and fail to release it fast enough. Myler's confident penalty reduced the arrears and re-ignited the crowd.
Malone coolly steered home a drop-goal to make it 22-14 in the 70th minute but, when replacement Geraghty had a chance to cut back the scoreline, he hooked his penalty.
However, referee Davey gave him another chance six minutes from time - a poor decision for a supposed high tackle - and Geraghty made no mistake this time to pull the hosts back within five points.
Ashton then took centre-stage with a well-taken try under the posts in the final move of the match to draw the game level, and Geraghty calmly slotted the extras to give his side a narrow win.
Tries: Dowson, Ashton
Pens: Myler 3, Geraghty
For London Irish:
Pens: Malone 4
Drop goal: Malone
Northampton Saints: 15 Ben Foden, 14 Chris Ashton, 13 Jon Clarke, 12 James Downey, 11 Paul Diggin, 10 Stephen Myler, 9 Lee Dickson, 8 Roger Wilson, 7 Phil Dowson, 6 Courtney Lawes, 5 Juandre Kruger, 4 Ignacio Fernandez Lobbe, 3 Euan Murray, 2 Dylan Hartley (captain), 1 Soane Tonga'uiha
Replacements: 16 Brett Sharman, 17 Jon Vickers, 18 Brian Mujati, 19 Christian Day, 20 Neil Best, 21 Alan Dickens, 22 Shane Geraghty, 23 Chris Mayor
London Irish: 15 Delon Armitage, 14 Tom Homer, 13 Elvis Seveali'i, 12 Seilala Mapusua, 11 Peter Hewat, 10 Chris Malone, 9 Paul Hodgson, 8 Chris Hala'Ufia, 7 Steffon Armitage, 6 Kieran Roche, 5 Bob Casey, 4 Nick Kennedy, 3 Paulica Ion, 2 David Paice, 1 Clarke Dermody
Replacements: 16 Danie Coetzee, 17 Dan Murphy, 18 Faan Rautenbach, 19 Andy Perry, 20 Jonathon Fisher, 21 John Rudd, 22. Ryan Lamb, 23. Alfredo Lalanne
Referee: Sean Davey