[Yes, I know the ball can be in without touching the line if it’s on the inside, just couldn’t think of a better, succinct title.]

Currently, the Nordea Open clay tournament is being played in Båstad, Sweden. They have a Hawkeye challenge system in place, of the ‘real bounce’ type where you see a super-slow-motion black-and-white shot of the ball landing, recorded by an actual camera, rather than just computer graphics.

A couple of days ago, I was watching a match (I think it was Albot vs Rune, but I’m not sure), and one of the players challenged a close call.

The slow-replay showed quite clearly that even at the maximum point of contact – when the ball is compressed the most vertically and has its widest horizontal area of contact with the ground – no part of the ball actually touched the line. It was equally clear from the side shot, however, that the ‘outermost’ part of the ball as a whole was right above the very edge of the line.

In other words, if viewed from above, the circular outline of the ball would obscure perhaps a millimetre of the line; but when viewed from the side, you could see that the part that obscured the line was not in contact with the ground.

Based on the slow-replay, I assumed that the ball would be called out, but it was in fact called in, and the simplified graphic showing the outline of the ball and the line showed the ball as just obscuring part of the line (as if seen from above, not the side).

All references I can find, such as Official Tennis Rules, specifically mention the ball touching or hitting the line, which in this instance it did not.

So on what basis was the ball called in by the VAR system here, then? Does ‘touch’ in practice actually refer to obscuring from above (whether or not there’s physical contact)?

  • 1
    IIRC Hawkeye is acknowledged to not be perfect and you'll have to check the tournament rules on the usage of VAR and whether the chair has the authority to overrule it.
    – pboss3010
    Commented Jul 15, 2021 at 19:16
  • Hawkeye is also usually not used on clay - unless that's changed in the last year or so.
    – Joe
    Commented Jul 16, 2021 at 23:11
  • @Joe The commentators were talking about there being some difference between tournaments, with the French Open not using it, but Nordea Open opting to use it, so I think it’s on its way in on clay. Commented Jul 19, 2021 at 8:04

1 Answer 1



For a ball to be in, it must actually touch the line. Being above a portion of the line at point of maximum impact is not sufficient.

As you rightly mention, the rules are fairly clear. From the 2021 Friend at Court document:

If a ball touches a line, it is regarded as touching the court bounded by that line.

Similarly from your source (though I don't know how official it is, regardless of the name of the website):

If a ball touches a line, it is considered to be the same as hitting the space inside that line.

Contrast this with association football, in which the relevant rule is:

The ball is out of play when it has wholly passed over the goal line or touchline on the ground or in the air.

Of course, a major difference is that in tennis we consider only the moment when the ball hits the court, whereas in football a ball can go out of play high in the air. Regardless, a similar rule for tennis could be along the lines of "If a ball is not wholly past a line when it touches the ground, it is regarded as touching the court bounded by that line". As this is not the rule, I argue the actual rule is clear in requiring the ball to physically make contact with the line.

You mention that the graphic system showed the outline of the ball. It shouldn't. A Hawk-Eye-like system is supposed to (attempt to) show the part of the ground that has come into contact with a ball. This also explains the sharp oval shape of the output of Hawk-Eye, as the area of the ball touching the ground first increases and then decreases, all the while the ball moves forward. See for example here for a short discussion and demonstration of this effect. Note that this is also different from showing the area touched by the ball at point of maximum contact.

So why was the ball called in?

Being unfamiliar with the specific game, I see a couple possibilities for why the call in a situation as you describe it might be called as in:

  1. The ball touched the line. This might seem silly, but if even some of the extending fibers (or hairs) of the ball touch the line, it is technically in. This can be very hard to see.

  2. The referee was wrong. Referees are human, and as all humans do make mistakes all the time.

  3. The automated system was wrong. Hawk-Eye has some error rate. Although it is likely a lot better than humans at determining in real time whether a ball is in or out, it is not capable of sub-millimeter precision.

  • I am under the impression that HawkEye has error of up to 4 mm ("average error of 3.6 mm" is commonly quoted, but is ambiguous in meaning).
    – Nij
    Commented Aug 1, 2021 at 0:37
  • 1
    In this case, I suspect that the issue is less the error rate and more the definition. Does Hawk-Eye, regardless of the official rules, treat a ball that, viewed from above, is over the line as in bounds even if it never touches? I rather see that as the question being asked. I find it very unlikely that Hawk-Eye has any way to see if a strand of fiber touched the line. It analyzes video. It should be possible for a human being to analyze the same video, although there may be gaps as the ball may bounce between frames.
    – mdfst13
    Commented Aug 1, 2021 at 7:22
  • This pretty much confirms what I thought was the case, except for the addition about the ball moving as it lands and giving an oval footprint (ballprint?), which I hadn’t considered. In this case, the sideways high-speed video view showed the ball in a similar position to the illustration in the physics discussion you link to – a little further from the line, perhaps, but very similar. I too highly doubt that any Hawk-Eye system is able to detect fibres extending from the ball (since those are definitely sub-millimetre), so I can only conclude that the system and referee were both wrong. Commented Aug 1, 2021 at 9:20
  • 1
    @mdfst13 The definition Hawk-Eye uses is certainly the correct one. However, it also certainly cannot detect every fiber, for resolution and frame-rate reasons if nothing else. The system works by creating a model for the ball in terms of position, velocity and spin over time, and using this model to create the statistically most likely "footprint". This model likely incorporates the fiber of the ball as simply a slight increase in the size of the ball. Importantly, it does not simply take overhead video to determine the overlap between ball and line, which would be incorrect w.r.t. this rule.
    – ADdV
    Commented Aug 1, 2021 at 10:38
  • A similar, but even clearer, instance of this just happened on the deciding point in fifth game of the second set of the Luxembourg Open WTA tournament final between Clara Tauson and Jelena Ostapenko. Tauson challenged, the super-slow-motion showed the ball touching the ground about 5 cm outside the line – but the system called it in, and that was the call that stood. The discrepancy here was so large and so obvious that Tauson complained to the referee who verified with the Hawkeye people that IN was the system’s actual call. Commented Sep 19, 2021 at 21:32

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