How to judge a good shot in tennis

Score the shot, not the outcome.

Key facts

1
Expected Shot Value measures a shot as the change in the probability of winning the point, estimated from the shot's measured features.
Floyd, Hoffman & Fokoue (2020), Journal of Quantitative Analysis in Sports
2
A shot taxonomy built from years of Australian Open tracking data found shot type strongly associated with winning, with the most effective shots in the highest-speed, lowest-net-clearance categories.
Kovalchik & Reid (2018), Journal of Sports Sciences
3
Time to net, a Game Insight Group metric, captures a ball's effective speed after spin and arc, so two serves at the same speed can reach the opponent at different times.
ESPN on the Game Insight Group's Australian Open metrics
4
Commercial metrics such as ATP Shot Quality don't disclose how they're calculated.

Why it works

A ball's value is how much it changes the chance of winning the point. That chance moves with what the ball does to your opponent: how far back it pushes them, how little time it gives them, how far it makes them run.

Depth and speed steal time. Angle steals position. Low net clearance is the sign of a ball hit hard and flat with margin to spare. A 120 mph forehand into the net is worse than an 80 mph one painting the corner, yet the box score counts both the same.

Winners and errors judge the last shot by what happened next. A winner off a bad ball is your opponent's error. Rate the ball at the bounce, and let the reply be your opponent's story.

The line to say on court
“Rate the ball at the bounce. The next shot is the opponent's story.”

Drill: Ten balls, rated blind

15 min · alone

Watch any match with pause allowed. Pause at the bounce of a rally ball and say its depth (short, mid or deep) and its height over the net (low or high).
Predict whether the reply will be weak. Play on and check.
Rate ten balls. Count how often a deep, low ball produced a short reply.

Number to beat7 of 10 deep, low balls that forced a short reply

More drills like this: watch like an analyst

Faults and fixes

Rating a shot by its speed

Speed is the least predictive of the three things you can see.

FixCall the depth before you let yourself notice the pace.

Rating a shot by its outcome

A winner off a bad ball is the opponent's error.

FixRate the ball at the bounce, before the reply.

Treating the danger zone as fixed

It shifts with the surface and your opponent's position.

FixJudge depth and height against where your opponent is standing.

Questions players ask

Is depth or pace more important in tennis?

Judge depth first. Of depth, net clearance and speed, speed is the input the eye overrates. A slower ball that lands deep and crosses low can force the reply from behind the baseline.

What is expected shot value in tennis?

It's the change in the probability of winning the point caused by a shot, estimated from the shot's measured features.

Why isn't a winner always a good shot?

A winner can come off a bad ball because your opponent missed their chance. Judging by outcome credits you for their error.

What does low net clearance tell you about a shot?

A ball that crosses low was hit hard and flat with margin to spare. The tracking research found the most effective shots in the highest-speed, lowest-clearance categories.

What this doesn't cover

True shot quality needs ball speed and spin measured at scale, which a viewer can't see. Rating depth, height and speed by eye is a proxy. Commercial shot-quality metrics are undisclosed, and the danger zone shifts with surface and opponent position.

Sources

Kovalchik & Reid (2018), Journal of Sports Sciences: a shot taxonomy from Australian Open tracking data
Floyd, Hoffman & Fokoue (2020), Journal of Quantitative Analysis in Sports: expected shot value
Kovalchik et al. (2020), arXiv:2005.12853
ESPN on the Game Insight Group's Australian Open metrics
Match Charting Project

StrikeWhen to hit flat and when to hit topspin in tennisHit flat when the ball is short and low and you're inside the baseline. Hit topspin when you're deep and the ball is high. One racquet-head speed buys either a flat drive or a heavy ball, never both. Tilt the swing path upward and you trade forward speed for spin, which gives you more margin but lands shorter.Know the gameHow does Hawk-Eye work in tennis, and how accurate is itHawk-Eye works by filming the ball with about ten high-speed cameras, crossing their sight lines to fix its position frame by frame, and fitting a flight path through those points. The bounce mark is computed from that path, not photographed. Its claimed mean error is about 3.6 mm, inside the 5 mm tolerance the ITF requires.Know the gameWhat makes a big point in tennis: win probability and leverageTennis win probability is each player's live chance of winning the match, computed from the score and how often each wins points on serve. Leverage is how much one point moves that chance, and it's what makes a big point. A 30–40 break point at 5–6 in a deciding set carries huge leverage. 40–0 at 3–0 carries almost none.Play the pointWhat is percentage tennis and how do you play itPercentage tennis means choosing targets and shots so the point ends on your opponent's error more often than on yours. It isn't pushing. It's aggressive with margin: aim through the net window, land past the service line, and go to the bigger half of the court and the weaker wing. About two in three points end on an error.