Coach Paul Roos knows if Sydney repeats last week's below-par effort against the Kangaroos when it faces St Kilda on Saturday night its six-game AFL winning streak will come to a shuddering halt.

After making a fast-start against the Roos at Manuka Oval, the Swans went to sleep and allowed the struggling Melbourne side to build a 32-point buffer midway through the third term.

Only some late heroics from co-captains Barry Hall, who re-claimed top spot in the Coleman Medal hunt with six goals, and Brett Kirk allowed the Swans to keep their league-best streak alive and hang onto fourth spot.

But Roos, whose team has managed to forge a solid 7-3 record after a 1-3 start, knows a similar performance against St Kilda will lead to a first defeat in seven matches.

"We can't afford to come out as lethargic as what we did in the first half last week otherwise we'll be a long way behind," Roos said before training on Thursday.

"St Kilda have got good power forwards in (Nick) Riewoldt and (Fraser) Gehrig and have a great midfield, so if we're not on our game it won't be 32 points down, it'll be 60 points down and the game will be all over.

"We definitely need to come out a lot stronger in the first half then we did last week."

St Kilda has once again been rocked by injury this season and lost key midfielder Lenny Hayes to a year-ending knee injury last week.

But Roos felt the likes of Luke Ball and Nick Dal Santo had picked up their games in Hayes' absence and performed solidly in Sunday's 13-point loss to the in-form Demons.

Dal Santo finished with three goals and 29 touches, while Ball had one of his best games of the season after a year blighted by injury.

Roos also conceded his side may have been mentally fatigued last week after performing at such a high level the previous five weeks.

"It was probably more mental (than physical) because they did finish off the game well," he said.

"If we were in front and tailed off you'd say it was more physical, but it is hard.

"Every week is very difficult mentally and physically and it's very hard to continually win in this competition because it's so even.

"In the end we deserved to win. They were probably the better team for more of the game than we were, but when it was there to be won it was a good effort from the players."

Sydney is keen to continue its winning run with a tough month ahead, where it will face fellow top four sides Adelaide, Collingwood and West Coast, as well as the unpredictable Dockers.

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