Some liquids, if cooled rapidly enough to avoid crystallization, can be frozen into a nonergodic glassy state. The tendency for a material to form a glass when quenched is called “glass-forming ability”, and is of key signiﬁcance both fundamentally and for materials science applications. Here we consider liquids with competing orderings, where an increase in the glass forming ability is signalled by a depression of the melting temperature towards its minimum at triple or eutectic points. With simulations of two model systems where glass-forming ability can be tuned by an external parameter, we are able to interpolate between crystal-forming and glass-forming behavior. We ﬁnd that the enhancement of the glass-forming ability is caused by an increase in the structural diﬀerence between liquid and crystal: stronger competition in orderings towards the melting point minimum makes a liquid structure more disordered (more complex). This increase in the liquidcrystal structure diﬀerence can be described by a single adimensional parameter, i.e. the interface energy cost scaled by the thermal energy, which we call the “thermodynamic interface penalty”. Our ﬁnding may provide a general physical principle for not only controlling the glass-forming ability but also the emergence of glassy behavior of various systems with competing orderings, including orderings of structural, magnetic, electronic, charge and dipolar origin.