Jankovic was due to play China's Zheng Jie in a match between losing semifinalists Team Europe and Team Asia Pacific, but appeared on court in a green track suit to inform the crowd at Victoria Park Center Court she had withdrawn and apologize.
"I have been sick for the last couple of days. I feel very weak,'' Jankovic told the crowd, adding that she has seen a doctor and received medication.
Speaking to reporters later Saturday, Jankovic said she had a fever Friday night and wouldn't say if she expected to be fit for the Australian Open, which starts on Jan. 19 in Melbourne.
She said she may not even catch a booked flight to Melbourne on Saturday night if she didn't feel well enough.
"It's all individual. Some people might recover in two days. Some people take longer. Who knows?'' Jankovic said.
"It's a long way coming here to Hong Kong. My immune system is down as well because I've been training very, very hard. I reached my limit. It's easy to get sick, especially being surrounded by many people and a change of climate, change of time zone and everything,'' she said.
Playing in Hong Kong as her only preparation for the Australian Open, Jankovic lost to Venus Williams 6-2, 6-2 on Thursday.
She appeared sluggish, with her serves lacking pace and her ground strokes erratic.
Teaming up with Portugal's Michelle Larcher de Brito, she also lost a doubles match to Zheng and India's Sania Mirza 6-1, 6-1 on Friday.
"It's obvious I feel slow and I have no reactions,'' Jankovic said Saturday.
After losing to Williams she said is typically a slow starter at the beginning of the season and is getting used to her stronger physique after a new fitness training program during the offseason.
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