nginx upstream server path "backend.example.com/serverfiles/"?

nginx upstream server path "backend.example.com/serverfiles/"?

I was wondering how to define server value in upstream to directory path
instead of subdomain or port...
for example this won't work :
upstream backend {
server backend1.example.com/**dir/** ;
server backend2.example.com:8080;
}
server {
location / {
proxy_pass http://backend;
}
}
Does anyone have a clue how could i do this?
Best regards.