The Blues reached last year’s final, where they were hammered by Barcelona, but needed a late equaliser from Harder to avoid defeat and rescue a 3-3 draw in their opening group game against German side Wolfsburg.
Erin Cuthbert had given Chelsea the lead after half an hour but it lasted just six minutes before Barbara Bonansea volleyed the hosts level.
Harder’s strike midway through the second half was enough for victory to send the Blues into the double-header against Servette level on points with group leaders Wolfsburg, and a point ahead of third-placed Juventus.
Harder and Sam Kerr returned to the team after being on the bench in the win against Leicester at the weekend, while Melanie Leupolz and Ji So-yun were also recalled.
A fast start from the hosts saw Lina Hurtig scoop a shot over the crossbar early on, before Bonansea put a header just over.
Chelsea had enjoyed plenty of the ball without really threatening Juventus keeper Pauline Peyraud-Magnin, but that changed after half an hour.
Harder found Cuthbert in space out on the right and she cut inside to score with a fine solo effort from inside the penalty area.
However, the lead was short-lived. Lisa Boattin spotted Bonansea’s run and her cross was volleyed home after 36 minutes.
There was a huge let-off for Juventus just before half-time. Peyraud-Magnin sliced her kick straight to Kerr but her shot was narrowly off target with the goal gaping.
Harder restored Chelsea’s lead after 68 minutes with her fifth goal of the season in all competitions, finding the back of the net through a crowded penalty area following a neat one-two between Fran Kirby and Kerr.
Former Arsenal goalkeeper Peyraud-Magnin produced a fine save to deny Kerr shortly afterwards as Chelsea looked for a third goal.
Juventus applied some late pressure as they searched for an equalising goal but Emma Hayes’ side held on in the stadium where the final will be staged.