From what we can see, the two vehicles feature closely related front ends with similar headlights. The prototype of the Prius fighter did feature a hockey stick-shaped band of LED driving lights in its front fascia; an element that's missing from this car.
Further similarities can be found where the windscreen, bonnet, front door and front fenders all meet up.
It's towards the back of the vehicle that the two prototypes begin to diverge. The car seen here features a flatter roof line, a traditional upright hatchback and a taller set of rear doors.
Hyundai's Prius competitor, meanwhile, has a much more a aggressively raked roof in the name of aerodynamic efficiency. It also features a Prius-style liftback tailgate.
Both the earlier prototype and this vehicle were spotted near Hyundai's R&D centre in Germany.
Last week Hyundai released teaser sketches of its next-generation Elantra sedan, which it says will be unveiled "soon" and will eventually replace the current-generation sedan that was unveiled in 2010.
The i30 that's on sale today made its debut in 2011. Despite featuring unique body panels and interior design, the i30 and Elantra are mechanically very closely related, with the i30 even being sold in the US as Elantra GT.