I have been there, twice in the late 1990's. You can e-mail me on firstname.lastname@example.org
for more details if you like. It is a very classy spa/resort, not cheap, but not overpriced. Family run. The resort itself is very European (except the tennis instructors - they were a mix of euros and Americans/Aussies). When we went (we were living in London at the time) they all asked us how we found out about them and were surprised to have visitors that were not from Switzerland, Germany or other local countries. That said, it is a beautiful locale, playing tennis with the mountains as a backdrop. If you get a room, try and go with another couple as the suites (2-bedrooms) are fantastic. The food is fantastic and dining is an event, somewhat dressy, reflecting the somewhat formal atmosphere. All-round top notch Austrian hospitality.
As for the tennis, nice clay, scenic, beer served courtside, great week-long Peter Burwash program for advanced beginner-intermediate. Advanced players (4.0+) may not find it overly challenging but they try and make up for it. I am a 4.5 but the others with me were lower rated and we all had a great time.
Hope this helps.