Angelique Kerber sets up semifinal match with No. 1 Simona Halep

For anyone who doubted, this was Angelique Kerber in 2016 vintage form.

Kerber routed US Open finalist Madison Keys 6-1, 6-2 on Wednesday to reach the semifinals of the Australian Open, the tournament where she kicked off her career-best season in 2016 with a victory over Serena Williams in the final.

"I feel good here. This is a special one because I won here, my first Grand Slam," said Kerber.

Kerber's semifinal opponent will be top-ranked Simona Halep, who recovered from an early break to win nine straight games and defeated No. 6 Karolina Pliskova 6-3, 6-2 later in the day.

Kerber, 30, also won the 2016 US Open and finished that year with the No. 1 ranking. But she was upset in the first round at Melbourne Park last year, and her ranking dropped into the 20s.

She won the Sydney International to prepare for the first major of 2018 then won her first three rounds in Melbourne in straight sets.

Her third-round victory over Maria Sharapova meant she was the only singles Grand Slam winner remaining in the women's draw. But a challenging fourth-round win over No. 88 Hsieh Su-wei had commentators wondering if Kerber was in 2016 or 2017 form.

Against No. 17-seeded Keys, she responded by winning the first four games to take control of the quarterfinal -- her first at a major since the 2016 US Open. Keys held in the fifth game of the first set, but Kerber went on another roll to lead 3-0 in the second set.

Keys slowed the momentum by holding and then getting her first service break of the match, but Kerber responded quickly to finish it off in 51 minutes. She broke Keys six times in total and now has a 7-1 record against the American.

"I'm just trying to enjoy myself again on court," Kerber said of her change of fortunes. "I know that I was working hard in the offseason, and I know that I can play good matches. I know that I can win close matches and also, yeah, going out there and playing good in the bigger tournaments.

"I just try to find the feeling back that I had, like 2016, and just enjoying my time. I mean, on and off court."

Kerber takes no comfort from the 5-4 head-to-head lead on Halep despite winning five of the past six matches. The latest of those was a 6-4, 6-2 win at the 2016 end-of-season WTA championship.

"I think it will be a long match with a lot of long rallies," Kerber said. "I played against her for a lot of times, and it was always tough and close."

For Keys, it was another disappointing loss in a major. She lost the US Open final to Sloane Stephens in September but said she was learning with every experience in the majors.

"I'm not going to walk away from this tournament and think it was terrible because I had one bad match," Keys said. "I definitely think I have taken a lot of steps in the right direction and feel good about my game and feel like I'm thinking a lot clearer out there. I think it's just going to take a little bit of time."

The Associated Press contributed to this report.