She will face No. 1 Simona Halep, who defeated Angelique Kerber 6-3, 4-6, 9-7.
The second-seeded Wozniacki appeared to be coasting against the 22-year-old Mertens, getting breaks in the middle of each set. But she began getting tight when she was serving for the match at 5-4: From 30-15, she double-faulted twice and Mertens passed her with a forehand winner in between to make it 5-5.
The No. 37-ranked Mertens lifted, holding her serve at love and then having two set points in the next game before Wozniacki eventually held a game that lasted almost eight minutes.
Wozniacki dominated the tiebreaker and put memories of her last Australian Open semifinal behind her — she had match points against Li Na in the semifinal in 2011 but was unable to convert.
This time, experience helped.
“It means so much to me. I got really tight at 5-4. I kind of felt head against the wall,” she said. “I knew I had to try and stay calm.
“Once she had set points. I thought, ‘OK, now you just have to go for it she’s nervous now too.’”
Mertens, who trains at Clijsters’ academy in Belgium, was appearing in the semifinals on her debut at Melbourne Park and in only her fifth Grand Slam tournament. She had won 10 matches in as row after successfully defending her Hobart International title two weeks ago.
She took out No. 4-seeded Elina Svitolina in straight sets in the quarterfinal but was struggling to match a consistent Wozniacki until late in the second set.
Wozniacki’s win moves here a step closer to potentially regaining the No. 1 ranking, a position she hasn’t held in six years.
In a men’s quarterfinal late Wednesday, Roger Federer got cranky at the chair umpire for a technology flaw, using the rare emotional outburst as motivation.
The 36-year-old Federer, now the oldest semifinalist in Melbourne in 41 years, beat longtime rival Tomas Berdych 7-6 (1), 6-3, 6-4 and will next face a challenge from the next generation.
That will be against 21-year-old Hyeon Chung, the first South Korean to reach a Grand Slam semifinal and the youngest to reach the last four at a major since 2010.
Federer’s victory extended his winning streak to 14 in Australian Open quarterfinals and to nine in a personal rivalry with Berdych that dates to 2004. The 19-time major champion leads that head-to-head contest 20-6, including all five meetings at Melbourne Park.
Federer had to overcome a shaky start, dropping his opening service game and uncharacteristically questioning chair umpire Fergus Murphy because of a technological fault.
With Berdych serving for the first set in the ninth game, Federer had challenged a line call.
After a lengthy delay, Murphy called the control room and confirmed the replay graphic couldn’t be displayed on the stadium screen, and also that the original decision stood. When he added that Federer had no challenges remaining for the set, Federer approached the chair and the crowd cheers intensified.
“Yeah, but you can’t steal my challenge,” Federer told Murphy. “Do you feel comfortable with this? You’re OK with it?”
Seven points later, he eventually broke Berdych to get back on serve, and then won the tiebreaker. The match was as good as over.
“I had to get a bit lucky. A bit angry. A bit frustrated maybe at the umpire,” Federer said. “Anyway, glad to get out of that first set. It was key to the match.”
Federer later said he just wanted an explanation from the chair, and agreed that blowing off steam helped his cause.
The 58th-ranked Chung is the lowest-ranked man to reach the Australian Open semifinals since Marat Safin in 2004.
With Chung already through, and Kyle Edmund playing No. 6 Marin Cilic in the other half of the draw, it’s the first time since 1999 that multiple unseeded players have reached the Australian Open semifinals.