The Doi-Edwards (D-E) theory for entangled polymer dynamics is extended to account for polymer chain stretching in fast flows. Our analysis is based on a new estimate for the non-equilibrium size of the tube under a macroscopic deformation and yields an optimum primitive chain contour length [\E . u\(1/2)] times greater than the equilibrium (no-flow) value. This finding implies that polymer chains subjected to nonlinear step strain will only retract partially and should therefore maintain significant chain stretch up to the time the primitive chain escapes the deformed tube. Primitive chain retraction is also shown to reduce the number of entanglements between a deformed polymer chain and its neighbors, yielding significant enhancement of molecular relaxation at high deformations. Various steady-state and transient stress predictions based on this partial strand extension concept are compared with existing experimental data.