The Fano line shape

arising from the interference of pathways for the excitation of discrete and continuum states

plays a fundamental role in many branches of physics

chemistry

and materials science. Exciting the resonance with a high harmonic provides naturally a phase delay between the pathways leading to a complex asymmetry parameter. We demonstrate that its amplitude and phase can be controlled on the femtosecond and attosecond time scales

respectively. With our high-energy-resolution (10-meV) experiment

we dynamically image a resonance-enhanced electron wave packet during its temporal evolution

extracting both the amplitude and the phase. Calculations reproduce our experimental results. Our approach constitutes a method for measuring the photoionization delays of a resonance and enables the reconstruction of the electron wave packet in the time domain. This concept of an interference-controlled Fano line shape is a step toward attosecond quantum optics with potential ramifications into nanoscience and next-generation optical materials.