It usually means your toss is too low.
HOW TO STOP YOUR SERVE FROM GOING INTO THE NET
It’s easy to develop bad tennis habits. It’s even easier to forget the good advice you’ve been given. The first thing you need to know is that this happens to every player, at every level. The second thing you need to know is that you can do something about it, and do it quickly.
WHAT’S BROKEN: My serve keeps going into the net
HOW TO FIX IT:
If your serve goes into the net, it usually means your toss is too low, says Rick Macci, who has trained the Williams sisters, Jennifer Capriati and Andy Roddick, among others. A low toss limits your swing and wrist snap and forces you to push the ball. A slightly higher toss can help, or the solution might be to use the same toss but hit the ball at a higher point, rather than letting it drop (you should hit the ball when it’s at its peak or just below it). You should also adopt a Continental grip if you haven’t already done so. “If you have a frying-pan grip [semi-Western or Western], you’ll hit down and into the net often,” Macci says.
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