Tennis is one of the safer sports, as it involves no contact with other players, except for the very rare bumping into your partner in doubles. Tennis does involve a lot of running though, often at full speed, and players should take care to avoid the two main hazards underfoot: loose balls and slick spots.
Stepping on a Ball
Loose balls on the court are a common cause of serious tennis accidents. Many players leave balls on parts of the court where they are likely to run, perhaps thinking that they will remember where the balls are and avoid them. Even in casual play, it's easy to forget where a loose ball is. In a competitive match, the ball you're trying to hit demands all of your attention, and especially if a loose ball ends up behind you, making sure to avoid it while chasing and hitting the ball in play would require almost superhuman powers. In one of the most common scenarios, a first-serve ball hits the net, then rolls out eight feet or so, and the server assumes either he won't come to the net or, if he does, the ball will still be in front of him where he can see it. On one of these occasions, however, he's going to move farther forward than the ball on the ground, such as to close in on a volley, and then get lobbed, at which point the loose ball will be behind him as he's backing up--and looking up--to try an overhead.
Stepping on a tennis ball can sprain or break an ankle, break another part of the leg or the foot, and cause a fall that could result in further injury. Some such injuries can take months or even years to heal, if ever. It's just not worth the risk. Walk a few steps to pick up the ball or move it flush against a fence. Taking a few seconds and getting a tiny bit more exercise won't do you any harm, and if other players complain, either persuade them or, if they can't see reason, find new people to play with. Show your regard for your playing friends by insisting that they move dangerous loose balls from their side of the court just as you do from yours. Always stop play immediately if a ball rolls in from another court, and ask your opponents to do the same if they see a ball that you don't. Friends don't let friends step on loose balls!
Slick Spots on the Court
Tennis court surfaces vary greatly in how quickly they get too slippery when wet.
A clay court is often safe after as much as half an hour of drizzle, except for the lines, which can get quite slick. A player usually knows exactly where the lines are on a court, though, which helps her avoid stepping on them, and if she steps across, instead of along the line, enough of the foot may be on the much less slippery clay to retain traction. If a slip begins along a line, but the foot slides onto clay before the slip has gone too far, the player might regain her balance. Experienced clay-court players are used to dealing with slick lines to some extent, as the lines offer considerably less traction than the clay even when dry. All of this said, the lines on a clay court can be dangerous when wet, and many players would be wise to avoid playing under such conditions.
How quickly a hard court becomes too slippery when wet depends on its speed. A slow hard court has more sand in the paint and thus a rougher texture that provides more traction. A short sprinkle that wets only half of the surface area of each square foot might not make play unsafe on a slow hard court, while the same rain would likely make play clearly unsafe on a fast hard court. On most hard courts, the lines will become slippery sooner than the rest of the court, because the line paint usually doesn't have its own sand, so the extra layer of paint smooths over the sand underneath. Some players might be able to judge whether or not a hard court is safe by testing the traction with their shoes, but doing so accurately takes considerable experience, and if in doubt, the wise choice is to wait until the court is completely dry.
It usually takes only a minute or two of drizzle to make a grass court too slippery to play.
For those of you who enjoy playing in cold weather, be extremely careful on a court that's very slightly wet; it may be safe while the temperature is above freezing, but if those bits of water turn to ice, they'll be much more dangerous.
Wetness isn't the only slipping hazard. The sand in hard-court paint wears away over time and, along with sand and silt from outside the court, can collect in patches where it's dense enough to become quite slippery. Pine needles, leaves, cut grass and other such debris can also be slick. A broom or even a snow brush can usually clear such hazards quickly. Very dry clay courts and badly worn grass courts can become more slippery than usual, but the slippery areas are usually large and thus less surprising.
If you love playing tennis, nothing could be more foolish than endangering your future ability to play by risking injury from a loose ball or a slippery court. The time it takes to move loose balls, clear sand or debris, or even to wait for a dry court is nothing compared to how long it might take you to recover from an on-court accident.