Police said the 19-year-old victim was approached from behind by an unknown man near Chiswick railway station – he is believed to be around 40 years old.
She was punched in the back of the head, after being knocked to the ground. She was continuously punched as she lay on the floor, then rushed to hospital when found.
The man is said to be around 6ft tall, white, and of a broad build.
Anyone who witnessed the assault or has information should call Hounslow CID on 101.
To give information anonymously contact Crimestoppers on 0800 555 111 or visit.