The new physics reach of null tests with D → π ll and Ds → K ll decays

vertical bar Delta c vertical bar = vertical bar Delta u vertical bar = 1 processes are unique probes of flavor physics in the up-sector within and beyond the Standard Model (SM). SM tests with rare semileptonic charm meson decays are based on an approximate CP-symmetry, a superior GIM-mechanism, angular distributions, lepton-universality and lepton flavor conservation. We analyze the complete set of null test observables in D -> pi ll(()'()) and D-s -> K ll(()'()) decays, l(()'()) = e, mu, and find large room for new physics safely above the SM contribution. We identify signatures of supersymmetry, leptoquarks and anomaly-free U(1)'-models with generation-dependent charges, for which we provide explicit examples. Z'-effects in c -> u ll(()'()) transitions can be sizable if both left-handed and right-handed couplings to quarks are present.