Yes , Lord Balaji was Lord Vishnu. Tirupati balaji temple located in chittoor district of Andhra Pradesh. It is one of the richest temple of india.
Story of lord Balaji :
once there was a debate among the sages as to who among the trinity should get the fruit of their sacrifice.
For this work, sage Bhrigu was appointed. sage Bhrigu went to lord Brahma but Brahma was busy with his wife Saraswati so he didn't notice Bhrigu. in fit of anger, Bhrigu cursed Brahma that he would not be worshipped on the earth.
Then Bhrigu went to lord shiva but shiva also busy with his wife Parvati. When Bhrigu disturbed shiva then shiva got angry. Seeing this, Bhrigu cursed shiva that he would be worshipped in form of ling.
After visiting shiva and Brahma, Bhrigu visited to vaikuntha the abode of Vishnu where lord vishnu was sleeping and didn't notice Bhrigu. In fit of anger , Bhrigu kicked on the chest of lord vishnu in front of goddess Lakshmi the wife of Vishnu.
Vishnu waked up and hold the foot of Bhrigu and asked that is he in pain by kick. Happy with Vishnu’s humble, Bhrigu said you deserve the fruit of yagya.
But goddess Lakshmi got angry with Vishnu and leave vaikuntha because Vishnu didn't punish Bhrigu for his action.
Goddess Lakshmi hide on earth and started penance. Lord vishnu searched everywhere but he cannot find her. So in grief and solitude he took shelter in an ant hill.
Seeing this incident, lord Shiva and Brahma took form of cow and calf and took shelter on the kingdom of king yadav naresh. When cowherd used to take cow to graze then cow used to gave milk to Vishnu.
Once the cowherd saw the incident then he throwed an axe toward cow. to save cow Vishnu appeared but axe hit on vishnu's head.
In fit of anger, Vishnu killed the cowherd. Vishnu cursed the king that he would become the danav in next birth but when king ask sorry to Vishnu then Vishnu gave boon to king that he would marry his daughter that’swhy he took incarnation of lord srinivas or Balaji. It is said that lord Balaji took loan from kubera for marriage and promised that he would pay in the end of kaliyug.