Dementieva had more chances at 6-5, twice pegging Williams back to deuce, but the world number two clinched it with a forehand passing shot which had her screaming towards her support camp and punching the air in delight - and, no doubt, relief.
That proved to be the turning point.
Williams broke in the next game, and reached the final for the fifth time when Dementieva was forced wide.
Centre Court was half-empty as Venus and Safina entered the arena, the crowd taking a break after the drama of the first semi-final.
Many may not have returned in time to catch a glimpse of five-times champion Venus, who made just one unforced error in a near-flawless display.
Safina lost the first eight points and won only eight points in total in the second set.
"She's just too good on grass," said the Russian.
"It's not my favourite surface, and it's her favourite surface. I think she gave me a pretty good lesson today."
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