Shimano 10 Speed STI vs 9 Speed rear Derailleur

[Seeker911]

I have a 9 speed triple dura ace groupset on my roadie. The right hand STI (rear derailleur) has started giving me issues and wont change down. I can buy a set of 10 speed Ultegra STI's from CRC for a very good price. The question is what else needs to change ?

 

Off the bat I assumed that I would have to change the rear cassette, rear derailleur and chain all to 10 speed. Ok so I have a brand new 10 speed ultegra cassette at home so that leaves the chain and rear derailleur. Someone mentioned that they did not think I would have to change the rear derailleur meaning I simply need to buy a chain and the 10 speed STI's.

 

So the question is will a 9 speed dura ace rear derailleur work with a 10 speed Ultegra STI ? if so will I be limited to 9 gears or will I get 10 gears ?

[Zeps]

I use a DuraAce 9sp RD with 10speed setup on my TT bike with no issues ..... all gears work 100%