WASHINGTON - Former world number one and No 10 seed Angelique Kerber from Germany needed three sets and almost three hours to beat world No 125 Wang Yafan from China to reach last eight at the Miami Open on Monday.

Finding herself just a game away from defeat in the second set, the relentless Kerber gathered up her steam to defeat the qualifier 6-7(1), 7-6(5), 6-3. With the victory, Kerber extends her excellent form this season, having now reached the quarterfinals or better at every tournament she's played in 2018.

"I think Wang played good, and for me, it was not so easy to find my game," Kerber said. "But also, I never played against her. I had no idea what to expect. I just knew that she played good here the last few days and also few weeks."

"But for me it was not so easy, and I was just trying to fighting. I think that was the key for me, that I was really fighting for every single ball, also especially in the second set in the tiebreak when it was 5-5," the German added.

The match was almost close as Wang broke six times while Kerber broke seven times and the total points scored by each player were almost the same, with Wang claiming 117 points compared to Kerber's 128.

Kerber will take on No.13 seed Sloane Stephens of the United States, winner of last year's US Open, in the quarterfinals of the event.

"For me it will be a challenge," Kerber said of Stephens. "I mean, the last time I lost against her. Now I just can try again to going out there, play my best, and trying my best to give all I can to win the match."

Kerber is bidding to become the first left-handed player to win the Miami Open since Monica Seles in 1991, and the first German to win the title here since Stefanie Graf in 1996.