I don't know if anyone has mentioned this yet or not so there's a link, and you can find many more. My achilles hurt for 1 1/2 years and I tried everything.
After 3 weeks of eccentric's it was noticeably better and now after 5 months I
can't tell I ever had a problem, and I only do them once a week. I never layed off tennis but didn't do much else, now I am back up to 18 miles a week running with no problem.
Oh, let me add : I only do one eccentric exercise. On a seated calf machine I push the weight with my good leg and let it off with the other all the way. I use enough weight to resist with
a very slow rep. And I do three sets of this along with 3 sets of regular calf rises with both legs.
Good Luck !