Ko, 24, has missed the past two events after hurting her ankle at the Taiwan Swinging Skirts LPGA on 1 November.

This week's winner at Tiburon in Florida, USA, will receive $1.5m (£1.16m), the highest single prize in the history of women's golf, and be crowned Race to the Globe champion.

"I'm still worried," said Ko.

Speaking before the Tour Championship starts on Thursday, the South Korean said she had only practised "maybe just one or two days on Monday and Tuesday".

She added that she still has some pain when she walks.

Ko will be the tour's player of the year regardless of who wins the event, which runs until 24 November, after winning four tournaments, including two majors.

The Race to the CME Globe is a season-long competition in which players accumulate points in every LPGA tournament, and the top 60 gain entry into the Tour Championship finale.

The player who wins the Tour Championship will be named Race to CME Globe champion.

England's Jodi Ewart Shadoff, Georgia Hall, Charley Hull and Bronte Law are among the 60 players who will tee off in Florida.