How to counter Sylas
Let us take a look on how to beat Sylas. We will look at champions who are strong against Sylas and who are weak against Sylas. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Sylas. You can already win in champion select with the right counters for Sylas. If you want to know how to counter Sylas then you need to take into account the best picks vs Sylas and the worst picks vs Sylas. This Sylas counter data is based on matches played by Pros on patch 14.21 and previous ones. It is important to see whether Pros counter picked Sylas or whether they played as Sylas.
Champions used by Pros to counter pick Sylas
Looking at the matches from Pros (see table Pros counter picked Sylas) we can see that they picked 
Yone most often to counter pick Sylas. The matchup Yone vs Sylas occured 81 times. Sylas has 44 wins and 37 losses in this matchup. Learn how to counter pick Yone to get another perspective on the data.
Lane outcome for Sylas
When looking at Sylas 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 Sylas. The lane outcome of Sylas consists of three outcomes: Win, Loss or Draw. Let's take a look of Sylas 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 Sylas.
Conclusion
Analyze the available data and learn how to beat Sylas. To master Sylas take also a look on Sylas builds and runes for Sylas. Good luck, Summoner!