On the eve of her Australian Open title defence, Caroline Wozniacki admits the biggest unknown in 2019 might be the sport's most-enduring champion Serena Williams.
Williams will gun for her 24th major title this year at Melbourne Park, returning to Australia after missing last year's tournament with the birth of her first child.
Wozniacki, who claimed a maiden slam in Williams' absence, is out for her second.
Williams' first-round clash with Tatjana Maria on Tuesday will be her first competitive match since the US Open final, lost dramatically and farcically to Naomi Osaka.
Aside from three matches and three wins in Perth at the Hopman Cup, where Williams' toughest opponent had been world No.49 Belinda Bencic, the 37-year-old has kept a low profile since her dramatic capitulation in New York.
Ahead of her own opening clash on Monday with Alison van Uytvanck, Wozniacki gave a frank assessment of where she might sit compared with the all-time great this month in Melbourne.
"I don't know," she said.
"I haven't seen her play a match either, so I don't really know. All I can say is that I feel good. I feel like I'm hitting the ball well."
At least Wozniacki, the world No.3, understands herself better than ever.
The Dane, 28, shocked the sport in October by revealing she suffered the debilitating autoimmune condition rheumatoid arthritis.
She responded by leaving the court behind in November, travelling to the Maldives, the British Virgin Islands and Anfield, home of her beloved football club Liverpool, before tuning back into the season.
"I took quite a bit of time for myself, just to relax and enjoy," she said.
"It was important for me just to leave the racquet behind for a while. That felt great ... I just really just listened to my body, just saw how everything was going.
"To be here as the defending champion, I'm just taking it as a nice, fun challenge.
"My first practice was on Rod Laver (Arena), and it was just nice. I felt comfortable straightaway when I hit balls on that court. It was just a good feeling."
"It's definitely great to be back."
Wozniacki was a shock second-round loser in the lead-up tournament in Auckland, but showed her readiness in a practice set earlier this week with world No.6 Elina Svitolina, winning 6-3 under match conditions on Margaret Court Arena.
Despite her ranking and performance last year, Wozniacki is ranked outside the top-10 chances at Melbourne Park by bookmakers.
Williams is seen as the most-likely winner, with 2016 champion Angelique Kerber next closest.
Aryna Sabalenka, Svitolina, Osaka and Karolina Pliskova are regarded as more likely winners, with Australian hope Ashleigh Barty also given a realistic shot at being the first local winner since Chris O'Neil in 1978.