diff --git a/lib/TicTacToe.js b/lib/TicTacToe.js
index 38e1d8f..956ac5c 100644
--- a/lib/TicTacToe.js
+++ b/lib/TicTacToe.js
@@ -218,13 +218,15 @@ class TicTacToe {
       }
     }
 
-    // check diag \
-    for (let i = 1; i < this.grid; ++i) {
-      let samePiece = 1,
-        prevPiece = null;
-      for (let j = 0; j < i + 1; ++j) {
-        const CURRENT_INDEX = this.grid - i + (this.grid + 1) * j - 1;
-        if (prevPiece === this.board[CURRENT_INDEX] && this.board[CURRENT_INDEX] !== NA) {
+    if (this.pieceWinCount == this.grid) {
+      // scans only the longest diagonals
+      let samePiece = 1;
+      for (let i = 0; i < this.grid - 1; ++i) {
+        const CURRENT_INDEX = i * this.grid + i;
+        if (
+          this.board[CURRENT_INDEX] !== NA &&
+          this.board[CURRENT_INDEX] === this.board[(i + 1) * this.grid + (i + 1)]
+        ) {
           samePiece++;
           if (samePiece === this.pieceWinCount) {
             this.winner = this.board[CURRENT_INDEX];
@@ -233,16 +235,15 @@ class TicTacToe {
         } else {
           samePiece = 1;
         }
-        prevPiece = this.board[CURRENT_INDEX];
       }
-    }
 
-    for (let i = 1; i < this.grid - 1; ++i) {
-      let samePiece = 1,
-        prevPiece = null;
-      for (let j = 0; this.grid - i - j > 0; ++j) {
-        const CURRENT_INDEX = i * this.grid + (this.grid + 1) * j;
-        if (prevPiece === this.board[CURRENT_INDEX] && this.board[CURRENT_INDEX] !== NA) {
+      samePiece = 1;
+      for (let i = this.grid - 1; i > 0; --i) {
+        const CURRENT_INDEX = i * this.grid + (this.grid - 1 - i);
+        if (
+          this.board[CURRENT_INDEX] !== NA &&
+          this.board[CURRENT_INDEX] === this.board[(i - 1) * this.grid + (this.grid - i)]
+        ) {
           samePiece++;
           if (samePiece === this.pieceWinCount) {
             this.winner = this.board[CURRENT_INDEX];
@@ -251,44 +252,82 @@ class TicTacToe {
         } else {
           samePiece = 1;
         }
-        prevPiece = this.board[CURRENT_INDEX];
       }
-    }
+    } else {
+      // scan all diagonals
+
+      // check diag \
+      for (let i = 1; i < this.grid; ++i) {
+        let samePiece = 1,
+          prevPiece = null;
+        for (let j = 0; j < i + 1; ++j) {
+          const CURRENT_INDEX = this.grid - i + (this.grid + 1) * j - 1;
+          if (prevPiece === this.board[CURRENT_INDEX] && this.board[CURRENT_INDEX] !== NA) {
+            samePiece++;
+            if (samePiece === this.pieceWinCount) {
+              this.winner = this.board[CURRENT_INDEX];
+              return this.board[CURRENT_INDEX];
+            }
+          } else {
+            samePiece = 1;
+          }
+          prevPiece = this.board[CURRENT_INDEX];
+        }
+      }
 
-    // check diag /
-    for (let i = 1; i < this.grid; ++i) {
-      let samePiece = 1,
-        prevPiece = null;
-      for (let j = 0; j < i + 1; ++j) {
-        const CURRENT_INDEX = i + j * this.grid - j;
-        if (prevPiece === this.board[CURRENT_INDEX] && this.board[CURRENT_INDEX] !== NA) {
-          samePiece++;
-          if (samePiece === this.pieceWinCount) {
-            this.winner = this.board[CURRENT_INDEX];
-            return this.board[CURRENT_INDEX];
+      for (let i = 1; i < this.grid - 1; ++i) {
+        let samePiece = 1,
+          prevPiece = null;
+        for (let j = 0; this.grid - i - j > 0; ++j) {
+          const CURRENT_INDEX = i * this.grid + (this.grid + 1) * j;
+          if (prevPiece === this.board[CURRENT_INDEX] && this.board[CURRENT_INDEX] !== NA) {
+            samePiece++;
+            if (samePiece === this.pieceWinCount) {
+              this.winner = this.board[CURRENT_INDEX];
+              return this.board[CURRENT_INDEX];
+            }
+          } else {
+            samePiece = 1;
           }
-        } else {
-          samePiece = 1;
+          prevPiece = this.board[CURRENT_INDEX];
         }
-        prevPiece = this.board[CURRENT_INDEX];
       }
-    }
 
-    for (let i = 1; i < this.grid - 1; ++i) {
-      let samePiece = 1,
-        prevPiece = null;
-      for (let j = 0; this.grid - i - j > 0; ++j) {
-        const CURRENT_INDEX = this.grid * (i + 1) - 1 + (this.grid - 1) * j;
-        if (prevPiece === this.board[CURRENT_INDEX] && this.board[CURRENT_INDEX] !== NA) {
-          samePiece++;
-          if (samePiece === this.pieceWinCount) {
-            this.winner = this.board[CURRENT_INDEX];
-            return this.board[CURRENT_INDEX];
+      // check diag /
+      for (let i = 1; i < this.grid; ++i) {
+        let samePiece = 1,
+          prevPiece = null;
+        for (let j = 0; j < i + 1; ++j) {
+          const CURRENT_INDEX = i + j * this.grid - j;
+          if (prevPiece === this.board[CURRENT_INDEX] && this.board[CURRENT_INDEX] !== NA) {
+            samePiece++;
+            if (samePiece === this.pieceWinCount) {
+              this.winner = this.board[CURRENT_INDEX];
+              return this.board[CURRENT_INDEX];
+            }
+          } else {
+            samePiece = 1;
           }
-        } else {
-          samePiece = 1;
+          prevPiece = this.board[CURRENT_INDEX];
+        }
+      }
+
+      for (let i = 1; i < this.grid - 1; ++i) {
+        let samePiece = 1,
+          prevPiece = null;
+        for (let j = 0; this.grid - i - j > 0; ++j) {
+          const CURRENT_INDEX = this.grid * (i + 1) - 1 + (this.grid - 1) * j;
+          if (prevPiece === this.board[CURRENT_INDEX] && this.board[CURRENT_INDEX] !== NA) {
+            samePiece++;
+            if (samePiece === this.pieceWinCount) {
+              this.winner = this.board[CURRENT_INDEX];
+              return this.board[CURRENT_INDEX];
+            }
+          } else {
+            samePiece = 1;
+          }
+          prevPiece = this.board[CURRENT_INDEX];
         }
-        prevPiece = this.board[CURRENT_INDEX];
       }
     }