Sydney coach Paul Roos hasn't drawn much from St Kilda's two most significant defeats this season - to the Swans in round 11 and Brisbane last week - ahead of Friday night's AFL semi-final at the MCG.

Instead, Roos and his players watched more of the Saints' better performances on tape this week.

They expect St Kilda will be much better than the team that was embarrassed by 80-points last week and had it first loss after 10 wins in June.

"We're expecting them to play very, very well tomorrow night," Roos said before his players held a light kick-to-kick session on the MCG on Thursday.

"So in that regard it's probably a little bit hard to plan for them because they were a little bit off when we played them last time and they won't be that tomorrow night."

While Sydney's round 11 win over the previously unbeaten Saints was lauded at the time it's a concern to Roos now as he said that game might be of more value to Saints coach Grant Thomas.

"We did well in the areas that you need to to beat St Kilda, that's not to say it's necessarily going to happen again," Roos said.

"Obviously we've done something right to beat them, but sometimes that can be the worst thing that can happen to you because we sort of showed our hand a little bit.

"You're always then debating, do you play the same guy on the fella he played on last time because league players are very proud - if they get beaten once they like to win the next time.

"So that can often cause a little bit of a problem and sometimes you can outsmart yourselves to be honest."

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