Best app to track Tennis activity?

I use indor running... As I do not run otherwise indoors...

There are CIQ Apps that track tennis play.  I have not used any of them so I can't speak to what they do or what they track.  If you want to use functionality resident on your watch, here is a long description of things to consider and options you have.

I play a lot of tennis.  There are basically a couple options to consider:

1. Do you want a GPS track of your time on court?  

2. Do you want HR details tracked specific to an activity or just the daily tracking that already takes place if you have HR enabled?

3. Do you want a Calorie count specifically allocated to an activity?

Let's touch on each of these to help decide what is best on the watch:

1. The GPS track on court will be close to useless.  You are moving around a small distance, at times less that the level of accuracy of the GPS.  The track will just be a massive jumble over the court and the distance provided is a rough guess at best.  Give it a try and you'll see what I mean.  Thus I have determined running the GPS is of little to no use when playing.  Instead I simply look at the distance covered based on steps and the accelerometer before I start, then again afterwards and use that as a more accurate measure.  More on this in a moment.

2. Your VA3 is already tracking your HR all day long if you have that enabled.  You an see it on Garmin Connect online at any time in the Daily Summary.  That is not associated to a specific activity, just the daily track.  So you have to decide if you want a specific activity logged with the HR in it also or not.

3. The system is already tracking calories to a very mild extent.  You'll see that on the Daily Summary as well.  It decides Resting vs Active by itself.  You running an Activity does not seem to affect it's decision.  So like #2, it's really a choice if you want to have a specific Activity logged with the Calories in that entry.

Wrapping it all together - Decision #1, do you want a specific Activity logged in your Garmin Connect account?  If so, then use a profile like Indoor Track.  That creates an Activity that will upload when done, it will have your HR trace and the watches guess on calories in it along with times, etc.  It does NOT turn on the GPS because it is an indoor activity.  So you don't consume battery but also don't have a distance.  If you really want the GPS's semi accurate guess at distance, use the Run or Walk Activity which will use the GPS.  After it uploads to Garmin, you'll need to edit it either online on Garmin Connect or on your phone app to chanse the activity to Tennis.  There is no profile that will automatically set the activity to Tennis.

If you choose not to bother with an Activity running, but still want an Activity logged in your Garmin Connect account for the purposes of seeing when you did what and tracking overall activities (this is what I do).  Simply enter a Manual Activity on Garmin Connect when you are done.  I enter time, distance, and the activity being Tennis.  There is a computation and resulting data field within the watch called "Distance Covered".  They don't make it obvious how to get to this  You can install a CIQ watch face or data field that displays it.  I use "KISS" watch face and it displays it as one of the fields.  I can then look at it before and after playing.  Not automated, but its simple as I'm not after tracking every detail of every activity like Tennis.  

Many thanks for the comprehensive reply.

I am looking for a combination of 2 & 3 - essentially I just want to track the basic details of the "workout" during my singles tennis sessions. Essentially, these are duration, calories and HR profile - it would be nice to have an idea of distance travelled during the session, but not essential. 

I'm currently using the "Cardio" setting, which seems to give me most of the info I am looking for, though the calorie count does seem a little optimistic!!

That will work.  Garmin's calorie counts are not all that accurate from what I can tell.  In some activities they are high, others they are woefully low.  

I use the cardio activity for tracking tennis.  Not really worried about steps but do like to track HR and calories burned that are attributable to an activity.  I play 4x/week and helps me keep track of calories burned - especially during summer months.  Lots more calories burned in the heat btw.

I have vivoactive 4. There is "My App" on ICQ Store which have profiles for many sports, including tennis, and it's working good. However, activities saved from this app (and can be found in "Garmin connect"), aren't included in weekly and monthly fitness email reports (only those from original Garmin activities are), so I'm disappointed here. 

Tennis is specific and from original Garmin activities most people (and some official guidance) agree that Cardio is activity to choose for saving tennis. Problem here is that there's no speed/tempo and distance recorded, unless you include GPS. And like mtsarpilot said, there's no much advantage of that (GPS) because of limited space to move (and so questionable distance measuring) and also extra battery is drained.

So, running activity (without GPS) is maybe best solution for getting best measurement of playing tennis, in terms of speed and distance. I would really like to here other tennis players how they find their best practices.

I use volleyball when I play pickleball. Not sure how parallel the activities really are but it's my best guess.