Exeter Chiefs overturned an 18-0 half-time deficit to clinch a crucial bonus-point victory at 14-man Northampton.
Saints shrugged off the 14th-minute red card shown to David Ribbans to deservedly lead at the break through two Rory Hutchinson tries.
Exeter stormed back as Sam Skinner, Stuart Hogg and Stuart Townsend all crossed, before Mikey Haywood extended Saints' lead again at 26-19.
Ollie Devoto's try got Chiefs on terms and Joe Simmonds' kick sealed the win.
It sets up a final-day showdown for a home semi-final berth, with Exeter hosting third-placed Sale Sharks, who will need a bonus-point win to pip the Chiefs.
A thrilling match - the perfect example of a game of two halves - exploded into life in the early minutes.
Dan Biggar twice set up Hutchinson in very different fashion - first with a clever delayed pass from close range, and then with a chipped kick forward which the centre darted on to and just controlled the touchdown.
But in between the two tries came the flashpoint which went on to define the match, as England hopeful Ribbans was sent off after a review with the TMO adjudged he was high on Luke Cowan-Dickie.
Northampton's tenacity and dominance at the breakdown continued to pay off and Biggar's penalty and drop goal extended the lead.
But they could not resist the waves of pressure that Exeter applied after the break, as Skinner bulldozed over and Hogg exploited the space of the extra man to reduce the deficit to 18-12.
Biggar's penalty kept Exeter at arm's length, and although Townsend's try again threatened Saints' lead, Haywood responded on the hour mark, going over in the corner after great work from Tommy Freeman.
However, Townsend created a try for Devoto minutes later and Joe Simmonds' conversion brought Exeter level - with Simmonds holding his nerve to kick a 25-metre penalty and nudge the Chiefs ahead.
Northampton forwards coach Phil Dowson told BBC Radio Northamptonshire:
"I think there's a lot of exhausted players out there. The front eight worked hard to a man and I'm very impressed with their efforts.
"I thought we managed the situation well as well as putting in all that effort - I don't think this side has ever been accused of not giving effort - but I thought we managed the situation as well. We scored points, we were clinical in what we were doing and we didn't give Exeter opportunities, which unfortunately we couldn't do in the second half.
"The crowd had a massive impact in generating energy and getting the boys up. We don't like losing at home regardless of the opposition but the fact they sang at the end of the game showed they appreciated the efforts."
Exeter boss Rob Baxter told BBC Radio Devon:
"The five points are the crucial thing, ultimately. I can't say the performance was the important thing from today because the first half was one type of performance and the second was completely separate.
"Firstly you have to congratulate Northampton. There was a great atmosphere even with a limited crowd here, what's more important in a way is getting rugby back on that positive curve.
"We're a good team, we're not second in the Premiership for no reason, and we had to just reset the plan, follow it hard and physical. In the first half we didn't do anything to plan, and that's something the lads will have to push themselves about."
Northampton: Freeman; Sleightholme, Proctor, Hutchinson, Naiyaravoro; Biggar, James; Waller (co-capt), Matavesi, Painter, Ribbans, Ratuniyarawa, Lawes, Ludlam (co-capt), Wood.
Replacements: Haywood, Iyogun, Heffernan, Moon, Coles, Mitchell, J Grayson, Dingwall.
Exeter: Hogg; Cordero, Slade, Whitten, O'Flaherty; J Simmonds (capt), Maunder; Moon, Cowan-Dickie, Williams, Gray, Hill, S Skinner, Kirsten, S Simmonds.
Replacements: Yeandle, Hepburn, Street, Lonsdale, Capstick, Townsend, H Skinner, Devoto.
Referee: Craig Maxwell-Keys (RFU).