reporting by AAP

Geelong has blown a golden chance to lock up a top four spot after letting a 33-point lead slip in an upset 18-point loss to St Kilda.

After Brisbane's one-point loss to Collingwood earlier on Saturday, the Cats only needed victory to seal a double chance in September — and they led by their game-high margin at half-time.

But an extraordinary third quarter, where St Kilda kicked six unanswered goals and seven in total to Geelong's one, got Ross Lyon's charges back in the game.

The Saints then seized control in the final quarter, kicking away to win 16.11 (107) to 14.5 (89) in front of 31,945 fans at Marvel Stadium.

It was St Kilda's fifth win in seven matches — an excellent finish to the season.

Geelong (56 points, 14-8) sit fourth, just two points ahead of Brisbane, and will need to beat West Coast at Karinda Park to lock in a top-four berth, while a home qualifying final has likely gone begging.

Skipper Jack Steele (28 disposals, two goals) and Rowan Marshall (28 disposals, 21 hitouts) were crucial to St Kilda's resurgence.

Josh Battle largely kept Jeremy Cameron (three goals) quiet, while Callum Wilkie (28 disposals, 15 marks) was busy in defence.

Jack Higgins (three goals) and Darcy Wilson (two goals, 25 disposals) popped up at crucial times as the Saints shared the goals around.

But Bradley Hill (back) was substituted late, while Saints young gun Mitch Owens (two goals) could come under scrutiny after catching Tom Stewart high midway through the final term.

Classy Cats defender Lawson Humphries (24 disposals) stood out with his sublime ball use, while Jack Bowes (31 touches) and Shaun Mannagh (two goals, 21 touches) were among their better players.

Geelong kept the Saints goalless in the first term and nudged out to a 28-point lead via a Patrick Dangerfield long bomb early in the second quarter.

Steele finally got St Kilda on the board four minutes into the second quarter.

But a rapid-fire burst of three Cats goals put the visitors in control at half-time.

The Saints exploded out of the blocks to boot four unanswered goals, including two to Wilson, inside the opening five minutes of the third term.

Tim Membrey kicked St Kilda's fifth on the bounce late in the quarter to level the scores, before Higgins gave the Saints the lead.

St Kilda led by six at three-quarter-time, and while the final term initially proved an arm-wrestle, five consecutive Saints goals put the game to bed.

The Saints (13th, 10-12) finish their season against Carlton next Sunday.