diff --git a/src/compiler/moduleSpecifiers.ts b/src/compiler/moduleSpecifiers.ts
index 1850220a41..339858a4ac 100644
--- a/src/compiler/moduleSpecifiers.ts
+++ b/src/compiler/moduleSpecifiers.ts
@@ -171,7 +171,7 @@ namespace ts.moduleSpecifiers {
         return match ? match.length : 0;
     }
 
-    function comparePathsByNumberOfDirectrorySeparators(a: string, b: string) {
+    function comparePathsByNumberOfDirectorySeparators(a: string, b: string) {
         return compareValues(
             numberOfDirectorySeparators(a),
             numberOfDirectorySeparators(b)
@@ -216,7 +216,6 @@ namespace ts.moduleSpecifiers {
         for (
             let directory = getDirectoryPath(toPath(importingFileName, cwd, getCanonicalFileName));
             allFileNames.size !== 0;
-            directory = getDirectoryPath(directory)
         ) {
             const directoryStart = ensureTrailingDirectorySeparator(directory);
             let pathsInDirectory: string[] | undefined;
@@ -228,10 +227,18 @@ namespace ts.moduleSpecifiers {
             });
             if (pathsInDirectory) {
                 if (pathsInDirectory.length > 1) {
-                    pathsInDirectory.sort(comparePathsByNumberOfDirectrorySeparators);
+                    pathsInDirectory.sort(comparePathsByNumberOfDirectorySeparators);
                 }
                 sortedPaths.push(...pathsInDirectory);
             }
+            const newDirectory = getDirectoryPath(directory);
+            if (newDirectory === directory) break;
+            directory = newDirectory;
+        }
+        if (allFileNames.size) {
+            const remainingPaths = arrayFrom(allFileNames.values());
+            if (remainingPaths.length > 1) remainingPaths.sort(comparePathsByNumberOfDirectorySeparators);
+            sortedPaths.push(...remainingPaths);
         }
         return sortedPaths;
     }
diff --git a/src/harness/virtualFileSystemWithWatch.ts b/src/harness/virtualFileSystemWithWatch.ts
index 5b29c8532e..25b8f3efe5 100644
--- a/src/harness/virtualFileSystemWithWatch.ts
+++ b/src/harness/virtualFileSystemWithWatch.ts
@@ -645,7 +645,7 @@ interface Array<T> { length: number; [n: number]: T; }`
                 }
                 else {
                     // root folder
-                    Debug.assert(this.fs.size === 0);
+                    Debug.assert(this.fs.size === 0 || !!this.windowsStyleRoot);
                     this.fs.set(path, folder);
                 }
             }
diff --git a/src/testRunner/unittests/tsserver/completions.ts b/src/testRunner/unittests/tsserver/completions.ts
index 1e50af8935..857f1b6ebc 100644
--- a/src/testRunner/unittests/tsserver/completions.ts
+++ b/src/testRunner/unittests/tsserver/completions.ts
@@ -118,5 +118,133 @@ namespace ts.projectSystem {
                 }
             ]);
         });
+
+        it("works when files are included from two different drives of windows", () => {
+            const projectRoot = "e:/myproject";
+            const appPackage: File = {
+                path: `${projectRoot}/package.json`,
+                content: JSON.stringify({
+                    name: "test",
+                    version: "0.1.0",
+                    dependencies: {
+                        "react": "^16.12.0",
+                        "react-router-dom": "^5.1.2",
+                    }
+                })
+            };
+            const appFile: File = {
+                path: `${projectRoot}/src/app.js`,
+                content: `import React from 'react';
+import {
+  BrowserRouter as Router,
+} from "react-router-dom";
+`
+            };
+            const localNodeModules = `${projectRoot}/node_modules`;
+            const localAtTypes = `${localNodeModules}/@types`;
+            const localReactPackage: File = {
+                path: `${localAtTypes}/react/package.json`,
+                content: JSON.stringify({
+                    name: "@types/react",
+                    version: "16.9.14",
+                })
+            };
+            const localReact: File = {
+                path: `${localAtTypes}/react/index.d.ts`,
+                content: `import * as PropTypes from 'prop-types';
+`
+            };
+            const localReactRouterDomPackage: File = {
+                path: `${localNodeModules}/react-router-dom/package.json`,
+                content: JSON.stringify({
+                    name: "react-router-dom",
+                    version: "5.1.2",
+                })
+            };
+            const localReactRouterDom: File = {
+                path: `${localNodeModules}/react-router-dom/index.js`,
+                content: `export function foo() {}`
+            };
+            const localPropTypesPackage: File = {
+                path: `${localAtTypes}/prop-types/package.json`,
+                content: JSON.stringify({
+                    name: "@types/prop-types",
+                    version: "15.7.3",
+                })
+            };
+            const localPropTypes: File = {
+                path: `${localAtTypes}/prop-types/index.d.ts`,
+                content: `export type ReactComponentLike =
+    | string
+    | ((props: any, context?: any) => any)
+    | (new (props: any, context?: any) => any);
+`
+            };
+
+            const globalCacheLocation = `c:/typescript`;
+            const globalAtTypes = `${globalCacheLocation}/node_modules/@types`;
+            const globalReactRouterDomPackage: File = {
+                path: `${globalAtTypes}/react-router-dom/package.json`,
+                content: JSON.stringify({
+                    name: "@types/react-router-dom",
+                    version: "5.1.2",
+                })
+            };
+            const globalReactRouterDom: File = {
+                path: `${globalAtTypes}/react-router-dom/index.d.ts`,
+                content: `import * as React from 'react';
+export interface BrowserRouterProps {
+    basename?: string;
+    getUserConfirmation?: ((message: string, callback: (ok: boolean) => void) => void);
+    forceRefresh?: boolean;
+    keyLength?: number;
+}`
+            };
+            const globalReactPackage: File = {
+                path: `${globalAtTypes}/react/package.json`,
+                content: localReactPackage.content
+            };
+            const globalReact: File = {
+                path: `${globalAtTypes}/react/index.d.ts`,
+                content: localReact.content
+            };
+
+            const filesInProject = [
+                appFile,
+                localReact,
+                localPropTypes,
+                globalReactRouterDom,
+                globalReact,
+            ];
+            const files = [
+                ...filesInProject,
+                appPackage, libFile,
+                localReactPackage,
+                localReactRouterDomPackage, localReactRouterDom,
+                localPropTypesPackage,
+                globalReactRouterDomPackage,
+                globalReactPackage,
+            ];
+
+            const host = createServerHost(files, { windowsStyleRoot: "c:/" });
+            const session = createSession(host, {
+                typingsInstaller: new TestTypingsInstaller(globalCacheLocation, /*throttleLimit*/ 5, host),
+            });
+            const service = session.getProjectService();
+            openFilesForSession([appFile], session);
+            checkNumberOfProjects(service, { inferredProjects: 1 });
+            const windowsStyleLibFilePath = "c:/" + libFile.path.substring(1);
+            checkProjectActualFiles(service.inferredProjects[0], filesInProject.map(f => f.path).concat(windowsStyleLibFilePath));
+            session.executeCommandSeq<protocol.CompletionsRequest>({
+                command: protocol.CommandTypes.CompletionInfo,
+                arguments: {
+                    file: appFile.path,
+                    line: 5,
+                    offset: 1,
+                    includeExternalModuleExports: true,
+                    includeInsertTextCompletions: true
+                }
+            });
+        });
     });
 }