Washington Freedom inserted Seattle Orcas into bat first and Saurabh Netravalkar provided the first breakthrough as he sent Nauman Anwar back with six runs on the board.
Quinton de Kock (24) then knitted a cautious 28-run stand with Ryan Rickelton (4) before Ferguson dismissed them in back-to-backs, reducing Orcas to 44-3 in 7.5 overs.
Klaasen then struck a 30-ball 51 with the help of four boundaries and three sixes, but could not get any support from the other end as the team bundled out for 124 in 19.4 overs.
Ferguson returned with bowling figures of 4-26, while Netravalkar claimed three wickets.
In response, Washington Freedom lost Travis Head on the second ball of their chase with Imad Wasim taking the prized wicket.
Rachin Ravindra scored 26 off 16 but Seattle Orcas bowlers picked wickets at crucial times to reduce Washington Freedom to 64-5.
However, Lahiru Milantha (33) and Obus Pienaar (30) constructed an unbeaten 63-run stand to ensure a thrilling last-over victory for Washington Freedom.
Nandre Burger took two wickets for Orcas while Imad Wasim, Cameron Gannon, and Harmeet Singh managed one each.
Washinton Freedom is comfortable sitting in first place in the MLC 2024 points table with an unbeaten record in three games, while Seattle Orcas are at the bottom with two points.