We have studied spin observables in the three-body break-up reaction in deuteron-deuteron scattering in the phase-space regime that corresponds to the quasi-free deuteron-proton scattering process with the neutron as spectator. The data are compared to measurements of the elastic deuteron-proton scattering process and state-of-the-art Faddeev calculations. The results for iT(11) and T-22 for the quasi-free scattering data agree very well with previously published elastic-scattering data. A significant discrepancy is found for T-20, which could point to a break-down of the quasi-free assumption. (C) 2013 Elsevier B.V. All rights reserved.</p