Sugiyama has not only reached the semifinals of more than one Tier II event, she's also won more than one Tier II tourney.
Anyway, the competition on the WTA Tour these days is so much stronger now than in the past two years when the Williams and Belgians were practically untouchable at Majors. I think Dementieva is certainly given renewed hope to see two lower-ranked players win surprise Slams this year. And right now she's playing well, so I expect at least a quarterfinal from her at the US Open.
Dementieva proved last night that her ground game is just as good as Serena's. What's holding her back is the serve. But she's still only 22 and has time to work on it, if she really focuses on it with her coach and takes time off from playing to improve that one shot. If she does, she will contend for Majors on a pretty consistent basis. If she doesn't, she still has the ability to win one (probably the French or Aussie which have slower surfaces and don't require good serves), as she showed this year by making it to the Final 2 in Roland Garros without a serve.
As for Dokic, I can't help you there. She's struggling right now and seems unmotivated, unfocused.