Geelong have blown the AFL finals series wide open with a gutsy 59-point upset win over premiership fancies Sydney.

The Cats played Patrick Dangerfield deep in attack and his four first-half goals spearheaded their 15.8 (98) to 5.9 (39) semi-final triumph on Friday night at the MCG.

It brought Sydney’s barnstorming run this year to a shuddering halt, while Geelong will now head to Adelaide for next Friday night’s preliminary final against the Crows.

It is also Sydney’s lowest score since round one, 1997.

After starting the season with a 0-6 losing streak, the Swans had won 15 out of 17 before Friday night.

The popular thinking was that the Swans would dispatch Geelong and play Adelaide in a titanic preliminary final, with the victor emerging as the premiership favourite.

Conversely, there were big question marks over the Cats, especially their hardness.

Since the 2011 premiership, they had only won two out of nine finals and they were staring down the barrel of another straight-sets September exit.

Already at long odds, the Cats suffered a major blow when key defender Tom Lonergan was a late withdrawal with food poisoning.

Lonergan, who retires at the end of this season, has a good record against Lance Franklin.

Despite a heavily bandaged right thigh, Franklin would have started the game thinking he had a big night ahead.

But instead Franklin had a nightmare, kicking 0.3, with Harry Taylor going back into defence and leading a superb team defence against the Coleman Medallist.

At the other end of the ground, Cats coach Chris Scott produced a masterstroke.

Scott took Dangerfield out of the midfield and played him one-out in the goalsquare.

The Cats last used the tactic when Dangerfield kicked a match-winning haul of 5.6 in round 17 against Hawthorn.

That was done out of necessity because he had a foot injury.

This time it was done because the Cats were so poor in attack last week against the Tigers.

Slow starts had killed Geelong in their past two games against the Swans, the round-20 loss at Simonds Stadium and last year’s preliminary final debacle.

But after the Swans kicked the first goal, Dangerfield booted the next two and the Cats clearly were up for the fight.

Just as Dangerfield starred at one end and Franklin was struggling at the other, the Cats were outstanding from the start in the midfield.

Mark Blicavs blanketed Swans captain Josh Kennedy, while Sam Menegola was prolific.

Before Friday night, every time Geelong led at quarter time this season they had won.

They had a five-point lead at the first change.

The Cats went into overdrive in the second term, kicking six goals to one and leading by six goals at the main break.

Daniel Menzel, a controversial omission from last week’s team, kicked the opening goal of the third term and the game was as good as over.

Sydney desperately counter-attacked, but Franklin’s bad miss from a simple set shot late in the third term summed up their dirty night.