MELBOURNE, Australia—Karolina Pliskova, who has reached the quarterfinals of the Australian Open, says that she has begun to realize that she can play her normal game during the Grand Slams.

Before the 2016 U.S. Open, she had won six WTA titles but had never reached the fourth round at a major, and has said she would get nervous at the big events. But then, at Flushing Meadows, she upset Serena and Venus Williams and reached the final, losing in three sets to Angelique Kerber. Now, after winning in Brisbane to start the season, the world No. 5 is into her second Grand Slam quarterfinal.

“Definitely the result I had in [the] U.S. Open was the breakthrough, especially on a Grand Slam,” she said. “I think this little bit counts, as well. Also, with my form coming this year—with winning Brisbane—I was really playing well there. I felt really good there."

In the third round in Melbourne, Pliskova came from two breaks down in the third set to defeat Jelena Ostapenko. In her other three matches, she has dominated.

"So here, one match, [a] little bit struggling,” she said. “Was also important just to get through this one, so everything is just, somehow, going [a] little bit my way. I'm feeling well, actually, even [if] not the best.”

Pliskova will face Mirjana Lucic-Baroni for a spot in the semifinals on Wednesday. The two have played each other tough. In 2015, Lucic-Baroni beat Pliskova in Toronto in three sets. Later that year, in Wuhan, Pliskova defeated Lucic-Baroni, 7-6 (4), 5-7, 7-6 (5).

"I know her pretty well," Pliskova said. "I didn't see much of her matches. I saw just the one, against [Agnieszka] Radwanska. I think she was hitting the ball really well, playing fast. These conditions probably suit her as well."