The Centurions managed a great escape on Saturday, beating Melville by two wickets at Tompkins Park without key players Josh Inglis and Jaron Morgan.
Chasing 208 for victory, Joondalup fell to 7-129, before Michael Swart (42) and Jake Zsilinszky (38) steadied the ship to record an important win over a fellow finals contender.
On Sunday, Centurions bowler Ty Hopes spun the side to victory with impressive figures of 5-11 to bowl Fremantle out for 92 at Iluka Sports Complex.
Garrick Morgan anchored the innings with 44 not out, as the Centurions passed the total in the 25th over.
Joondalup, which is third, hosts Midland-Guildford on Saturday from 11am.