On the eve of the second Ashes Test it was the English duo that made the difference with goals either side of the break stunning the Australian side, who had largely dominated the first hour of the game.
Chelsea, who only arrived in Japan on Sunday were up against it early on against a side who have already played once since landing in Okayama and are midway through their domestic season, and it showed with Jodie Taylor and Leena Khamis both going close in the opening exchanges.
But after weathering the Sydney storm Chelsea struck with ten minutes of the half remaining. A passing move involving Danni Buet, Aluko and Rosella Ayane eventually ended up at the feet of midfielder Coombs, who shifted the ball onto her left foot and unleashed a fantastic low drive into the bottom corner from 20 yards.
Sydney responded well and almost found a leveller, only for Taylor's header to bounce on top of the crossbar before being diverted away.
Japanese star and Chelsea skipper Yuki Ogimi crashed a free kick from the edge of the box against the bar and after the ball was only partially cleared Aluko kept her nerve to plant her shot in to the back of the net from 18 yards.
The Blues were in total control from then on and six minutes later it was 3-0 as recent recruit Rachel Williams played a delightful through ball in to the path of Ogimi, who showed why she is the running for the Ballon d'Or as she skipped round keeper Casey Du Mont and rolled the ball home.
Sydney refused to throw in the towel and pulled one back in the 75th minute as Renee Rollason's free-kick from all of 25 yards had enough pace and bend to beat the outsretched arm of Chelsea keeper Marie Hourihan.
There was then a twist in the tale as the referee controversially awarded Sydney an indirect free-kick eights yards from goal and Taylor expertly lofted the ball in to the back of the net to set up a grandstand finish with nine minutes remaining.
But Sydney were unable to complete the comeback and Chelsea will now head to Tokyo for Sunday's final where they will face Japanese double winners INAC Kobe Leonessa, who overcame South American side Colo Colo 3-0 in the other semi-final.