Long cracks in the near threshold regime can show extension characteristics which are similar to those of short cracks. One interesting feature is retardation at obstacles such as precipitates, the other one is crack extension along crystallographic planes. Both phenomena have been observed both for through-wall cracks and for surface cracks. This behavior is traced back to the crack tip field in the present paper. For this purpose, the three-dimensional microstructure in the vicinity of the crack tip was resolved by combining high resolution X-ray tomography and microstructure analysis in the SEM. These investigations yield the three-dimensional geometry of the crack faces, the surrounding grain structure including grain orientation, and the spatial distribution of the primary precipitates. The crack tip field in this three-dimensional volume element was then determined using the multiphysics simulation package DAMASK. A conventional phenomenological crystal plasticity constitutive law for face-centered cubic crystals was used to capture the behavior of the aluminum matrix, whereas linear-elastic material behavior was assumed for the precipitates. This analysis yields the stress-strain field in the vicinity of the crack tip as well as the amount of plastic deformation. The rotational part of the latter quantity can be compared directly to the grain misorientation measured in the EBSD analysis in the SEM, and is a good indicator of the most likely crack extension direction. It is shown that the characteristics of the crack tip field defined by grain rotation are in very good agreement with those of the crack paths found in the experiments.