How to counter Mordekaiser
Let us take a look on how to beat Mordekaiser. We will look at champions who are strong against Mordekaiser and who are weak against Mordekaiser. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Top champion who counters Mordekaiser. You can already win in champion select with the right counters for Mordekaiser. If you want to know how to counter Mordekaiser then you need to take into account the best picks vs Mordekaiser and the worst picks vs Mordekaiser. This Mordekaiser counter data is based on matches played by Pros on patch 15.13 and previous ones. It is important to see whether Pros counter picked Mordekaiser or whether they played as Mordekaiser.
Champions used by Pros to counter pick Mordekaiser
Looking at the matches from Pros (see table Pros counter picked Mordekaiser) we can see that they picked 
Rumble most often to counter pick Mordekaiser. The matchup Rumble vs Mordekaiser occured 6 times. Mordekaiser has 5 wins and 1 losses in this matchup. Learn how to counter pick Rumble to get another perspective on the data.
Lane outcome for Mordekaiser
When looking at Mordekaiser counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Mordekaiser. The lane outcome of Mordekaiser consists of three outcomes: Win, Loss or Draw. Let's take a look of Mordekaiser lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Mordekaiser.
Conclusion
Analyze the available data and learn how to beat Mordekaiser. To master Mordekaiser take also a look on Mordekaiser builds and runes for Mordekaiser. Good luck, Summoner!