diff --git a/djangorestframework/serializer.py b/djangorestframework/serializer.py
index b0c026753..5dea37e81 100644
--- a/djangorestframework/serializer.py
+++ b/djangorestframework/serializer.py
@@ -25,16 +25,9 @@ def _field_to_tuple(field):
 
 def _fields_to_list(fields):
     """
-    Return a list of field names.
+    Return a list of field tuples.
     """
-    return [_field_to_tuple(field)[0] for field in fields or ()]
-
-
-def _fields_to_dict(fields):
-    """
-    Return a `dict` of field name -> None, or tuple of fields, or Serializer class
-    """
-    return dict([_field_to_tuple(field) for field in fields or ()])
+    return [_field_to_tuple(field) for field in fields or ()]
 
 
 class _SkipField(Exception):
@@ -110,9 +103,6 @@ class Serializer(object):
         self.stack = stack
 
     def get_fields(self, obj):
-        """
-        Return the set of field names/keys to use for a model instance/dict.
-        """
         fields = self.fields
 
         # If `fields` is not set, we use the default fields and modify
@@ -123,9 +113,6 @@ class Serializer(object):
             exclude = self.exclude or ()
             fields = set(default + list(include)) - set(exclude)
 
-        else:
-            fields = _fields_to_list(self.fields)
-
         return fields
 
     def get_default_fields(self, obj):
@@ -139,9 +126,7 @@ class Serializer(object):
         else:
             return obj.keys()
 
-    def get_related_serializer(self, key):
-        info = _fields_to_dict(self.fields).get(key, None)
-
+    def get_related_serializer(self, info):
         # If an element in `fields` is a 2-tuple of (str, tuple)
         # then the second element of the tuple is the fields to
         # set on the related serializer
@@ -175,11 +160,11 @@ class Serializer(object):
         """
         return self.rename.get(smart_str(key), smart_str(key))
 
-    def serialize_val(self, key, obj):
+    def serialize_val(self, key, obj, related_info):
         """
         Convert a model field or dict value into a serializable representation.
         """
-        related_serializer = self.get_related_serializer(key)
+        related_serializer = self.get_related_serializer(related_info)
 
         if self.depth is None:
             depth = None
@@ -219,7 +204,7 @@ class Serializer(object):
         fields = self.get_fields(instance)
 
         # serialize each required field
-        for fname in fields:
+        for fname, related_info in _fields_to_list(fields):
             try:
                 # we first check for a method 'fname' on self,
                 # 'fname's signature must be 'def fname(self, instance)'
@@ -237,7 +222,7 @@ class Serializer(object):
                     continue
 
                 key = self.serialize_key(fname)
-                val = self.serialize_val(fname, obj)
+                val = self.serialize_val(fname, obj, related_info)
                 data[key] = val
             except _SkipField:
                 pass