The 伺 in 伺候 is pronounced cì, NOT sì. When 伺 means 'to spy on' or 'to watch for' (as in 伺机 sìjī), it's read sì. Same character, different reading depending on the word.
usage
伺候 can sound either respectful (caring for elderly parents) or sarcastic (这人真难伺候 — 'this person is so hard to please'). Context is everything.
