And in Sunday's other games, Nell Morris-Dalton kicked a career-high four goals to lift Carlton to a 27-point away win over Sydney, while Hawthorn celebrated Kaitlyn Ashmore's 100-game milestone with a 25-point victory over Brisbane in Dingley.
Fremantle copped an embarrassing 100-point hiding from North last year, but the Dockers looked set to cause an almighty upset after kicking three straight goals to take a one-point lead during the second quarter in windy conditions on enemy territory.
But with Riddell and Garner in top form, North turned on the afterburners to kick six goals to one from that moment onwards to seal the 9.4 (58) to 4.6 (30) win.
Riddell finished with 38 disposals, four clearances, one goal and 510m gained, while Garner was also destructive with 33 possessions, four clearances, one goal and 517m gained.
Eilish Sheerin (24 disposals, nine clearances, one goal and 529m gained) also had a day out for North on an afternoon when the Kangaroos finished with nine individual goal scorers.
"It was a good arm wrestle. Credit to Fremantle, they used the ball quite well," Garner told Fox Sports after the match.
"Their uncontested footy challenged us through that middle part of the game, but the girls did well to fight back."
Kiara Bowers battled hard for Fremantle on the way to 31 disposals and eight clearances, with Aisling McCarthy (26 disposals) providing support.
The result ensures North (8-1) finish the round on top of the ladder, while Fremantle (4-5) have slipped out of the top eight and will likely need to win at least two of their remaining three games in order to make the finals.
Fifth-placed Carlton (7-2) are just 1.7 per cent adrift of the fourth-placed Suns after defeating fellow top-four hopefuls Sydney 8.11 (59) to 4.8 (32) at Henson Park.
Morris-Dalton was the star of the show with 4.1, while Darcy Vescio kicked two goals.
The Blues will sweat on the availability of skipper Abbie McKay, who limped off with an ankle injury in the second half.
Montana Ham (27 disposals, six clearances, one goal, 558m gained) battled hard for Sydney and Chloe Molloy kicked 3.2 from 19 disposals, but the Swans couldn't recover after conceding four goals to one in the opening term.
The loss dropped Sydney (6-3) to seventh.
Over in Melbourne, out-of-form Brisbane threatened to pull off an upset against Hawthorn after taking a 19-7 lead into quarter-time, but the Hawks turned the match with a five-goals-to-nil second term to set up the 9.11 (65) to 6.4 (40) win.
Aine McDonagh kicked three goals from 20 disposals for Hawthorn in a powerful display, while former Lion Ashmore celebrated her special milestone with a stunning set shot from the boundary-line pocket in the second quarter.
The victory moved Hawthorn (5-4) into eighth spot, while last year's beaten grand finalists Brisbane (2-7) are officially out of the finals equation.
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