Grandmasters needs also to work more on their endgame knowledge

 
You don't have to be an opening expert to score a win. You don't even have to outplay your opponent in the middlegame.
You can wait to the endgame to outplay your opponent. This is the phase where even Super Grandmasters from time to time plays very weak.

Magnus Carlsen scores a lot of wins in the endgame from positions that should have been impossible to win if the opponent's had more knowledge about endgame patterns.

Let's take a look at the game Magnus Carlsen - Sergey Karjakin, Wijk aan Zee 2013, Round 8.     

The game with analysis by chess engine Houdini at the Wijk aan Zee tournament site, tatasteelchess.com (page Live games Round 8). A link to the game also at Chessgames.com