Northampton raced into a 17-0 lead before 26 points either side of half-time seemingly put Saracens in command.
A yellow card for Saracens' Marco Riccioni allowed Northampton to regain control as they moved six points clear at the top of the table ahead of Saturday's fixtures.
Freeman, George Hendy and Tom Pearson were Northampton's first-half try scorers, while Fergus Burke, Jack Bracken and Juan Martin Gonzalez crossed over for Saracens.
Hugh Tizard bagged a second-half try for Saracens before Pearson and Freeman added their second tries to put the hosts in the ascendancy.
Freeman completed his hat-trick before grabbing Fin Smith's kick to score his fourth try, while Max Malins crossed for Saracens' fifth try.
