It is important to drill the fundamentals into your game until you can do them in your sleep. But often players forget to practice specifically what they do in a match. For example, I know a player who likes to counterloop with his forehand. He spends a lot of time practicing counterlooping with counterlooping practice partners. But in matches he has trouble counterlooping against an opponent's first loop off underspin, which is usually done closer to the table than other loops, has a different arc, and usually more topspin.
A simple drill to practice against this would be to have a practice partner serve backspin, the player pushes it back, the partner loops, and the player counterloops. The partner doesn't play out the point; as soon as he finishes his loop, he reaches for a ball from a box. (It's an improvised version of multiball.) This matches what a player faces in a match, as opposed to just counterlooping, and it gives far more practice on this specific skill in a given time than just playing out points. Plus, your partner gets lots of practice looping against backspin!
So work on your fundamentals, but also look at what you actually do in a match - or need to do - and find drills that match that specifically, and perfect the skill.