Sydney FC went into the weekend's first semi final as clear favourites, with Canberra United, who had only just snuck into fourth, aware that they had their work cut out.

But a straight red to last-woman-standing Sydney defender Alesha Clifford for fouling Canberra's Ashleigh Sykes in just the 13th minute suddenly changed the odds.

A 39th-minute substitution of Linda O'Neil, who'd been suffering from a head cold and sore throat going into the match and who had exhausted herself running to cover the deficit, also added pressure to Sydney.

But they showed the greater hunger of the two teams and reshuffled to look the less likely of the two teams to be a player down.

Leading by example, captain Heather Garriock forced a turnover and found Sarah Walsh in space, who then forced goalkeeper Lydia Williams into a save with her long-range shot.

A Garriock cross then drifted past both Walsh and former golden boot Leena Khamis in the 43rd, with Sydney well aware that they could have in that moment taken the lead.

They took it just one minute later, though, and right on half time, when Kyah Simon threaded the ball through a line of players and Walsh capitalised.

Keen to build a buffer, Sydney started the second half with equal attacking momentum.

Walsh continued to trouble the defence with her blistering speed and natural striker Simon almost went one better than her assist when she nearly got an outstretched boot onto a Garriock pass.

Not to be left out of the action, fellow striker Khamis scored out of an innocuous-looking pass, heading in a long-range Servet Uzunlar free kick that floated in front of the goal.

Ashleigh Sykes had the best chance for Canberra when she attacked against the run of play. But she drifted wide instead of pulling the trigger and Sydney flooded the area and cleared the ball.

Cian Maciejewski subbed on in the 62nd to inject some fresh legs and focus into the flagging Canberra, who were struggling to contain the Sydney attacks.

But she couldn't turn the game around.

Garriock almost scored Sydney a third, match-sealing goal in the 75th when she muscled a player off the ball and pulled the trigger at a 45-degree angle.

Cathrin Paaske narrowly missed minutes later, when she couldn't quite keep her header from the Kylie Ledbrook corner below the crossbar.

Emily Van Egmond sprayed a shot high and wide when she had the space to claw one back for Canberra. Khamis then fancied herself for a second, but too skewed the shot over the crossbar.

Then Khamis, Walsh, and Garriock combined in the 84th to create Sydney's third goal. Khamis passed the ball to Walsh, who laid it back to Garriock, who toe-poked it into the top left corner.

She almost made it a double shortly afterwards, when Paaske chested down a corner to her outside the box and Garriock drilled it low and hard through the box of bodies to force Williams into a diving-and last-second-sighted-save.

Sydney should really have made it 4-0 when Paaske got on the end of a corner and put the ball in the goal mouth prime, but Williams got a slight touch on the pass and Michelle Carney kicked for the ball but only got air.

Canberra threatened during the three minutes of stoppage time, but Sydney held possession and their nerve to reserve their first W-League grand final berth.

The match now marks their 10th and confidence-boosting consecutive game without a loss.

The second semi final played out on Sunday, with the Central Coast Mariners hosting Roar Women in what turned out to be a physical on-field battle.

And, like the semi final that preceded it, the victors were forced to grind out a win with just 10 women on the pitch.

Both teams entered the half-time break scoreless after an evenly matched first half saw them exchange-but not score with-shots on goal.

McDonnell and Tameka Butt combined to keep Central Coast goalkeeper Jillian Loyden busy and Aivi Luik too had a brilliant long-range shot in the 34th.

US import Kendall Fletcher spotted Roar goalkeeper Casey Dumont off her line at the other end of the pitch and sent her scrambling with a long-range shot that barely sailed over the crossbar.

The second half started with equal pace, with Ellen Beaumont testing Loyden before Roar captain Clare Polkinghorne lost her footing in the 53rd and Michelle Heyman found herself in space, took a shot, but was easily contained.

McDonnell put Loyden under pressure by running down and deflecting a back pass clearance with her body, with quick-thinking Loyden then securing the rebound.

Polkinghorne, who had been getting forward to help the attack, chased down a loose ball and pulled the trigger to force Loyden into a far-post save.

Then Jo Burgess was downed outside the box and the unlikely Reuter stepped up to take the free kick.

Much to everyone's surprise, her shot floated up and over the wall and keeper and dipped in to record her first goal of the season and give the Roar a much-needed lead.

But the match was far from over, with McDonnell awarded her second yellow and subsequent red in the 68th for infringing the 10 metres required for a free kick and slowing down the play.

The Central Coast took advantage of the overlap, pushing forward and plugging away at the Roar defence.

They almost equalised in the 83rd, with only Lauren Colthorpe's desperate header denying Renee Rollason's floating cross.

Hearts were in mouths for the four minutes of stoppage time, but the Roar held on to record the win and book themselves a berth in their second consecutive W-League grand final.

They will meet Sydney in Sydney at 2.50pm EDST on Saturday, December 19.