Out of form and battling illness heading into the tournament, American world number one Serena Williams did not expect to clinch a sixth Australian Open title.

Williams extended her winning streak over Sharapova to 16 matches and claimed a 19th grand slam title in the process courtesy of a 6-4 7-6 (7-5) victory on Rod Laver Arena.

But the 33-year-old's form leading into the year's first grand slam, not to mention her health throughout the fortnight, led to question marks over whether she could add to her impressive tally of silverware.

Williams lost two of four singles matches at the Hopman Cup in Perth, while she was clearly plagued by a virus at Melbourne Park - only to deliver the goods when it really mattered.

"It feels really good to be sitting here as the champion," Williams told a media conference.

"I definitely didn't think I would be here in the beginning of the week or the beginning of the two weeks, but it feels pretty excellent.

"I was not feeling great. Every time I didn't feel great I started to feel better, and then the next day I got worse. There was a lot of ups and downs.

"I've been through so much the past week. I really, really didn't expect to win. I didn't expect to be here this long. I was walking down the hall yesterday and I was thinking, Wow, I'm still in the tournament.

"It's been a long time since I've been to the final here or the semi-final. It's been a long time coming. I was just really, really elated to have an opportunity to walk out on the final match."

Williams somehow managed to fight off illness against the Russian on Saturday.

The veteran was visibly distressed due to a coughing fit during a rain delay in the sixth game of the opening set.

Williams went back down the tunnel to receive treatment and returned to the court in devastating fashion.

"The doctor gave me a little cough syrup and sent me on my way. It was like a small, small, small amount just to suppress," she added.

"I was coughing so much. It was just forcing things out of me. So I was just trying to pull myself together."