This is just my own opinion, however it's been my experience that dogs chasing a tennis ball...they're QUICK! In many cases, the auto focus on even the most expensive lens out there will have trouble keeping up with them, particularly if they're moving towards you (or away from you)...and continuous focus doesn't do much good if the lens can't keep up to begin with. Here's how I usually deal with it...
First off, as others have already said, try using a smaller aperture...f/6.3 isn't
too bad, but for a quick moving little dog, I'd try to get that up to the f/10 range (depending on your distance to the dog). You said you're using a Nikon D800 so don't be afraid to bump up the ISO if you need to keep the shutter speed up. The D800 has a good ISO range and with the likes of Photoshop and such, it's pretty easy to clean up any noise you may get with the higher ISO...don't be afraid to take advantage of it.
Second...try pre-focusing the camera on a fixed point parallel to the dog. This works great at dog shows, however it can bit a little trickier with a dog on the beach (LOL). I might suggest taking a stick (or something else) that you can put just out of frame, but parallel to the dog's path towards you...focus the camera on the stick (or whatever), have the person throw the ball to that same distance in the dogs path (yea...also tricky), then when the dog hits that point, all you have to do is click. I've done this with my own mutts at the river or dog parks and such...I'll pre-focus the camera on a rock or something in the dog's path then click the shutter a hair before the dog passes that point. It takes some practice, but can produce some really good results.
Third, instead of having the dog run towards you, have the person throwing the ball toss it so the dog runs
across your field of vision instead. Fortunately when most dogs chase a ball, they don't (usually) tend to zig-zag a lot...they usually make a bee line straight at the ball. Since I'm too lazy to draw a diagram here (LOL), think of a "T"...you're standing at the bottom of the T and the dog is running across the top "cross" of the T...it may not produce as thrilling a shot as the dog running towards you, however it's (usually) A LOT easier to focus.
Last but not least - patience! In this case I'd dare to quote the old adage, "some days you get the bear and some days the bear gets you", LOL! Seriously...I've had days out at the dog park where every picture is a keeper and I've had some days where I've just wiped the whole stinkin' memory card. Perhaps the best advice I can really offer is simply to relax, have fun and let your dog be a dog...if you don't get the shot, it gives you a good excuse to take your friend out again

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Just some ideas to play with...good luck!