The partnership of Stephan Keller and Simon Colosimo in the centre of Sydney FC's defence may only be three matches old but it's played a big part in helping the Sky Blues get off to a great start in their 2009/10 Hyundai A-League campaign.

Sydney FC is undefeated heading into its round-4 clash with the Wellington Phoenix having conceded only two goals and having kept back-to-back clean sheets in its last two outings. Despite this, Keller believes there is plenty of room for the back four to improve.

"The clean sheet against Adelaide I think we had maybe ten minutes (in the game) where we could've conceded something - for the rest (of the match) we did very well. The same thing against the Central Coast where the last ten minutes we were really lucky," explained Keller.

The Swiss signing is confident that with a strong start to build on, Sydney's understanding in defence will only get better.

"You don't build a very good defence in three weeks, it takes time to really get settled in with each other perfectly," he said.

Colosimo believes the backline is well on its way to doing just that, and that his combination with Keller is improving all the time.

"We started off conceding a couple of goals from set pieces (against North Queensland) but we've definitely progressed. Each game we've got better and better and we'll be looking to improve again this weekend," he said. "It's good when you get along with each other and you like each other and certainly you respect each other," added Keller. "That's what's happening between all four (of us) including (Sung Hwan) Byun and Shannon (Cole) at the moment - they're also contributing a lot to our defensive performance."

"I think it's easy to settle into this team because all the guys are really helpful and really professional and trying to help you," continued the 30-year old. "Everybody is behaving that way and I appreciate it very much."

This Sunday it will be Wellington's strikers trying to break down the Sky Blue wall at the Sydney Football Stadium and Colosimo is looking forward to the challenge.

"They (Wellington) have had a pretty good start to the season," he assessed. "I think Leo Bertos is their most dangerous player. I know he hasn't scored yet but I know Leo from my time in Perth and I think he's one of their best."

The Trans-Tasman clash with the Phoenix will kick-off at the SFS at 5:00pm on Sunday evening.