Focus on developing and exploiting one particular stat, while not ignoring the others.
The key here is to not become a “Jack of all trades”, but good in all traits and excellent in one. For example, if all your stats are in the range of 20 points, make sure that at least one of your stats, like Backhand or Volley, or whatever your choice, hovering around 30 points. The game’s meta works in a way that rewards you more if you exploit your strongest stat.
Say all your stats are around 20, and you’ve matched up with a player who’s stats are around 15 but the forehand is 25. This player has a massive edge against you, even though you are averaging higher in stats, because he can exploit his forehand and you won’t be able to counter that. Neither do you have any strong stat to exploit. Now say if you change your gear, and have your stats at 15 but your backhand at 25. Now you are better placed to counter that player and have higher odds of beating that player.
Having said that, don’t ignore any one particular stat. Make sure all your stats are going up together. Many players I have seen have ignored Stamina, and most games they lose because even though their other stats are higher, they tire out too soon. The same is with players who have their Serve too low. Their serve gets countered easily and eventually opponents exploit that weakness.