Sergei Vlasenko said he thought Tymoshenko was dead when he found her in a hospital shower, RIA Novosti reported Friday.
"I'm not a specialist, but the situation is critical," Vlasenko said, adding that a doctor was brought in to treat Tymoshenko.
Vlasenko said Tymoshenko did not immediately recognize him when he found her.
Tymoshenko was sentenced to seven years in prison in October 2011 for abuse of office after negotiating a gas deal between Russian and Ukraine in January 2009.
She maintains her conviction was politically motivated by President Viktor Yanukovych.
Tymoshenko was moved to a hospital last year after complaining of deteriorating health in prison.