Fenix 6 HR does not work in tennis

Tennis is a high-impact sport (in terms of impact on your wrists), your watch will move a LOT while swinging a racket around. Especially with a big, heavy watch like the Fenix. The optical sensor can't get a good reading when it's moving around all over the place on your skin. So you'll either need to substantially tighten the strap so it stops moving around, or use Garmin's advice to wear an external HR sensor.

I don't know anything about Suunto or why it seems like it worked better for you, but I suspect it's probably because it was smaller and/or lighter so it didn't move around as much during tennis. The suunto may have also been giving you higher readings (perceived as "more accurate") even without good skin contact, due to an issue we call "cadence lock", which is when the sensor falsely believes that all its shuffling around against your skin is your pulse.

Optical HRMs are dependent on as little movement on the device as possible, as already mentioned tennis will cause the watch to move extensively and as such won't provide a reliable heart rate, to be honest I'm not even sure that the Suunto watch would provide a reliable metric when compared to a decent chest strap.

For my own use I always rely on the chest strap, it's far more reliable that any of my watches (past and present).

But the Polar OH1 arm band and pair it with your watch.  MUCH better, I am pro tennis player and this is what I do all the time.  Works perfect.

I used Fenix 6 and Suunto Spartan at the same time in tennis game. Suunto worked 100% and Fenix 6 shows that my heart rate is never over 110. HR can be measured from wrist in tennis. So problem is just in poor quality in Fenix 6.

Fenix 6 works well in other sports but waste of money for tennis.

Maybe someone has already found a good attitude for the sport of tennis?

I can confirm. I had fenix 5 fr645 and now fenix6 none of them is able to record correct whr during tennis (my polar m600 was) so I bought a polar oh1 and now I use it whenever is possible 

Would you post the tracks for comparison? That could just be enough for me to move to Suunto but I won’t consider the move without verifiable evidence. Ideally links to the activities on the interweb in Strava perhaps?

I'm possibly as unhappy as you with the lesser (compared to the 5xPlus) quality of optical WHR on the 6 series, the recommendation to get a strap is on point IMO.  I've got both a 6xProSolar and a 5xPlus (and currently wearing both on each arm for testing) and the 6 WHR performance has improved significantly after many many updates from Garmin.  The new model sensor is disappointing but adequate for me now. 

EDIT to add:  I use a garmin chest strap for "run" and "hike" activities to ensure accuracy and to ensure consistent VO2 Max calculation (run).

I use the 6 Pro with a MyZone chest strap and it tracks HR perfectly.

For what its worth.. I have a 6s, and I play tennis twice a week.   I use this watch, and it works perfect for tennis.  HR often gets 160+ bpm.  No issues at recording at all.  I also have used it with the arm band, and chest strap and its also accurate.  So, if yours is not working.. there maybe another reason.