The European championship bronze medallist was accidentally hit in the face by Polish athlete Zaneta Glanc who had mimed a throw while holding the discus as she sat in the athletes area next to the discus circle at Moscow's Luzhniki stadium.
"I was next to her. The Polish athlete was warming up and hit her in the face," Olympic champion Sandra Perkovic told Reuters.
"Her nose is broken. When I saw it, I ran over to help her. I took my towel to her and some water," added the Croatian who is favourite to take her first world title in Sunday's final (1615 GMT).
Semenova had to be patched up by officials under the blazing morning sun in the competition area.
The 31-year-old, sporting a dressing on the bridge of her nose to cover the cuts and bruises, bravely continued with qualifying but managed only one legal throw of 55.79 metres, well down on her season's best of 61.41, to finish bottom of the group.
After coming off the field, a clearly upset Semenova walked past waiting reporters carrying a bloodied towel to her nose, pausing briefly only to tell one she was going for medical treatment.
(Editing by Justin Palmer)
