Jankovic will take medical advice before deciding on Doha
Elena Dementieva and Jelena Jankovic have fitness concerns ahead of next week's WTA Championships in Doha.
Dementieva withdrew ahead of Thursday's second-round match against Polona Hercog in Luxembourg because of an inflammation on her right foot.
And Jankovic complained of illness after losing to Kazakh qualifier Zarina Diyas at the Kremlin Cup in Moscow.
Dementieva explained her decision was precautionary and said: "I'm really disappointed but can't take the risk."
The top seed beat Shuai Peng of China 7-5 7-6 in the opening round but said: "I was able to finish the match, but I the next day I felt worse.
"I've tried so many different things, but it just doesn't get any better. It's sad, because I feel so welcome here."
Meanwhile Jankovic said after her defeat inside an hour to Diyas, who only turned 17 last week: "I got an infection and have been on antibiotics for a week or so.
"Unfortunately I wasn't ready to play at my level. I was suffering on the court but I really wanted to be out there and didn't want to retire.
"I finished as well as I could, but obviously it was tough for me to focus on my tennis as I had a hard time breathing."
The Serb, a former world number one and currently ranked seventh, hopes to recover in time for the end-of-season championships, which start on Tuesday.
The WTA Championships involve the top eight players in the world competing in two round-robin groups of four, ahead of semi-finals and the final.
WTA CHAMPIONSHIPS QUALIFIERS
"Doha is a different type of tournament," she said. "You can lose a match and still win the whole tournament. So anything can happen, but for me the biggest concern is being healthy.
"I will see the doctors, take some blood tests and take some more medicine. We will see what happens and hopefully I can be ready in time."
World number one Serena Williams and number four Venus Williams have already withdrawn from Doha because of injury, while Justine Henin will also be absent as she has not played since Wimbledon because of an elbow problem.