The German is also unsure whether she will be able to play at Eastbourne next week due to problems with the left hamstring strain which forced her to retire during her third-round match in Madrid.
"It started slowly and then I really felt it yesterday (Saturday) evening. It is my left hamstring, the same one I had in Madrid," she told reporters.
Kerber had a disappointing claycourt season and went out in the first round at the French Open. She said the injury flared up again when she returned to the courts last week.
"It was OK and I started practising again after Paris and then I felt it again in the last few days. I don’t want to risk anything now, to do something (worse)," she said.
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Kerber is the latest big name to have pulled out of the Aegon Classic following Simona Halep's withdrawal with an ankle complaint.
Kerber is chasing an elusive first title this year after failing to get past the third round in her last five tournaments.
The 29-year-old has struggled to impose herself in recent months and her only appearance in a final this year was at the Abierto GNP Seguros competition at Monterrey in April, where she lost to Russian Anastasia Pavlyuchenkova.
The Eastbourne International tournament begins on June 25 with Wimbledon getting under way a week later.
(Additional reporting by Aditi Prakash in Bengaluru; Editing by Toby Davis)
