The German will meet Italian qualifier and world No.100 Camila Giorgi, who upset Poland's former world No.2 Agnieszka Radwanska 6-1, 6-2. She is guaranteed to break into the top 50 rankings for the first time on Monday.
The 2016 Australian Open champion is in superb form going into this year's event, having also won all four singles at the Hopman Cup, but knows she faces a big test against Barty on Saturday.
The Italian looked incredibly focused as she drilled 33 winners to 29 unforced errors and more self-assured in controlling her second deliveries.
Former world number one Angelique Kerber maintained her flawless record in 2018 as she saw off Camila Giorgi in straight sets to reach the Sydney International final.
Rory McIlroy reveals heart condition, but still expects to play
It is what will be driving the four-time Major victor when he kick starts his 11th season in the pay-for-pay ranks. I don't fear any of them. "I just don't feel that now", he said.
Former world number one Kerber fell out of the top 20 last year but has made a strong start to the year ahead of next week's Australian Open with victories over Lucie Safarova and Venus Williams this week. "I was working hard in the last few weeks", Kerber said.
It's just the beginning of the year.
"We have set up a ideal platform for me to work with this year to try and go deep in tournaments, whether it's in Australia or in Europe or in the States. I'm trying to be aggressive and play like I did the year before last".
"I think it's unbelievable for Sydney and for Australian tennis - we're gonna have an Aussie in the final, which is awesome", Barty said after her win at the Ken Rosewall Arena.
Heat a factor in del Potro's NZ final loss
Haase won his quarter-final 6-4, 6-4 against Germany's Peter Gojowczyk who had beaten the top-seed and reigning champion Jack Sock the previous day.
Barty overcame compatriot Daria Gavrilova in three sets, coming back from a set down to win 3-6 6-4 6-2.
"I shot myself in the foot with my serving in the first and didn't give myself an opportunity to be dominant on service games", Barty said.
Italian fourth seed Fabio Fognini set up a semi-final against qualifier Daniil Medvedev following a 7-6 (7-4), 6-7 (7-4), 6-2 marathon against Frenchman Adrian Mannarino.
Mertens will play Mihaela Buzarnescu of Romania, who has reached her first WTA final.
Patriot's Rex Burkhead (knee) officially inactive against Titans
A final decision on Burkhead's status will likely be revealed when the Patriots release their inactives prior to the game. Rex Burkhead appeared to be trending toward a return from a knee injury he suffered against the Pittsburgh Steelers .
Meanwhile, Juan Martin del Potro kept his Australian Open preparations on track with a 7-6 (4), 6-3 win over Russia's Karen Khachanov in the Auckland Classic on Thursday, setting up a semifinal meeting with David Ferrer.