A man has been stabbed while jogging near a lake in western Sydney before escaping to a fast-food restaurant to get help.

The man, in his 20s, was jogging about 5am on Monday at Lake Woodcroft in Blacktown when he was stabbed four times in the back.

He then managed to get to a take-away restaurant, where he was met by police and paramedics.

He was taken to Westmead Hospital in a serious condition, while police hunt his attacker.