An electronic ballast for driving a gas discharge lamp avoids mercury pumping in the lamp by adaptively changing an operating frequency of an inverter of the ballast when operating near high-end. The inverter of the ballast generates a high-frequency AC voltage, which is characterized by the operating frequency and an operating duty cycle. The ballast also comprises a resonant tank for coupling the high-frequency AC voltage to the lamp to generate a present lamp current through the lamp, and a current sense circuit for determining the magnitude of the present lamp current. A hybrid analog/digital control circuit controls both the operating frequency and the operating duty cycle of the inverter with closed-loop techniques. The control circuit adjusts the duty cycle of the inverter in response to a target lamp current and the present lamp current. To avoid mercury pumping, the control circuit attempts to maximize the duty cycle of the inverter when operating at high-end. Specifically, the control circuit adjusts the operating frequency of the inverter in response to the target lamp current signal, the duty cycle of the inverter, and a target duty cycle in order to drive the operating duty cycle toward the target duty cycle.