Steffi Graf has warned Angelique Kerber that Serena Williams remains a huge threat to her place as world number one in the WTA rankings.

Kerber enjoyed a breakthrough year in 2016, winning the Australian Open and US Open, as well as reaching the Olympic singles final at Rio 2016 and the decider of the WTA Tour Finals.

The German ended Williams' lengthy reign at the top of women's tennis in September and finishes the year as world number one.

But Graf, who saw Williams match her Open-era record of 22 grand slams at Wimbledon, says the American will be determined to come back strong next year.

"With Serena, she's definitely had a little bit of a harder year; she wasn't able to play as many tournaments as she might have wished to be ready for the grand slams," she WTA Insider at a Q&A on Sunday. 

"But she's too incredible of a player to be underestimated in the coming year.

"Consistency is what we're all striving for on the court; it's difficult with different surfaces, different time zones and different climates. It's a long year in tennis."

Until this season, Kerber had often flattered to deceive on her early career promise but Graf believes her attitude changed over the past two years, which helped her make the breakthrough in grand slams.

"I felt she always had the talent and the strengths; she's been working hard. She just needed that one win to get her mindset changed to where she could start believing in herself," she added.

"She was really able to do that in the last year or two, where she started realizing that she can beat top players. 

"That belief really changed her game around; she's been playing with a lot more confidence, and you can see that she's a different player on the court. 

"With each victory, it seemed like she got stronger and more comfortable with herself on the court."