Htaccess rewrite with %REQUEST_FILENAME condition - apache

Question

I stucked. I need to:
all request with domain.com/api/... rewrite to public/ directory
all others request rewrite to frontend/index.html
but if request is a file, then rewrite just to 'frontent/'
I had two htaccess files:
.htaccess
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_URI} ^/api/ [NC]
RewriteRule ^(.*)$ public/$1 [L]
RewriteRule ^(.*)$ frontend/$1 [L]
</IfModule>
frontend/.htaccess
<IfModule mod_rewrite.c>
RewriteEngine On
# Send Requests To Front...
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^ index.html [L]
</IfModule>
It was working but now I need to remove the one from frontend directory without loosing it's rules. I changed the main .htaccess:
<IfModule mod_rewrite.c>
RewriteEngine On
# Send request to backend
RewriteCond %{REQUEST_URI} ^/api/ [NC]
RewriteRule ^(.*)$ public/$1 [L]
# Send request to frontend
RewriteCond frontend/%{REQUEST_FILENAME} !-d
RewriteCond frontend/%{REQUEST_FILENAME} !-f
RewriteRule ^ frontend/index.html [L]
# Request for file
RewriteRule ^(.*)$ frontend/$1
</IfModule>
But it doesn't work. I think that this conditions are wrong:
RewriteCond frontend/%{REQUEST_FILENAME} !-d
RewriteCond frontend/%{REQUEST_FILENAME} !-f
because all request (others than api) are rewrited to frontend/index.html which i mentioned.
Could anybody show me where did I mistake?