Officers are investigating an incident which took place in Claypath, Durham City, at approximately 2.15am on Saturday, February 1.
An appeal, which includes a CCTV image of the woman, has now been issued to trace the potential witness following the attack.
A spokesman for Durham Constabulary said: “In what appears to be an unprovoked assault, the victim suffered a single punch to the left hand side of his face leaving a deep cut which needed stitches.
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“Officers investigating the attack would like to speak to a woman wearing a white dress who was seen in the area shortly afterwards and who may have been present at the time of the assault.”
Anyone with information should call PC Sean Dickinson on 101.