Kerber: It's been an up and down Australian Open

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World number one and Australian Open champion Angelique Kerber concedes it has been a topsy-turvy start to her title defence in Melbourne.

Kerber progressed to the fourth round of the grand slam at Melbourne Park by seeing off Krystyna Pliskova 6-0 6-4 on Friday.

Back on Rod Laver Arena, Kerber threatened to steamroll her Czech opponent after racing through the opening set in 20 minutes, until Pliskova offered some sort of resistance as the two-time major winner looked shaky at times in the second set.

Speaking to reporters afterwards, Kerber said: "It was better, actually. I was feeling very good from the first point, but of course matches like this are always tough to play against opponents.

"They are serving very well, and you have to bring a lot of balls back, moving good, and just going for it. But I'm happy about my performance."

Kerber's straight-sets win comes after battles against Carina Witthoeft and Lesia Tsurenko, with both contests going to a deciding set.

Asked to rate her performances during the first week, Kerber replied: "I think it was a little bit up and down.

"All matches are different for me. But I think it's good to have different matches and going through to that, to being, then, the second round.

"Let's see what the next challenge is. I had a good pre-season. I was working a lot on my fitness. It was good to know I can stay for two or three hours on court. But my legs are always really important for my game."

Next up for Kerber is American Coco Vandeweghe, who overcame Eugenie Bouchard earlier on Friday.

"I think it will be maybe a little bit similar than [my match with Pliskova], but of course Coco is a tough opponent," Kerber said. "She played good today [Friday]. I saw the match a little bit before my match. I know that she's serving good as well."