Once again I have to read that in one of the threads here.
It's simply not true. During Martina's career (which I consider to be over after AO 2002), there was exactly ONE match against either Venus and Serena where she "could not compete with them": the US Open semifinal 2001 against Serena, and Martina was already on her way down then. All her other losses against them were hard-fought matches in which she could indeed compete.
IMO there are two other players who are much more "responsible" for Martina's decline: Steffi Graf and Jennifer Capriati.
At FO 1999 it all began. After that Martina has never been as tough mentally as before. She proved that in the US Open final, where Serena had a major choke and Martina couldn't take advantage of it.
Her two GS losses against Venus in 2000 were definitely painful, but it would be ridiculous to say that she couldn't compete. Martina was inferior mentally, but that wasn't just against Venus, that was also against Mary Pierce (FO 2000) and others, and the main reason for that is the Graf-match.
Then, at AO 2001, she beats both Venus and Serena. She trounces Venus, something which Venus never did to her (and they should only have ONE more match, at Miami, which Venus won in two close sets), and she comes back against Serena from 2 breaks down in the 3rd set (again they should only have ONE more match, the already mentioned US Open semifinal).
Then, to everyone's surprise, she loses against Jennifer. And THAT was the beginning of the end.
After that Martina's downfall becomes more and more obvious. She loses against Clijsters, against Sanchez-Vicario, against Ruano-Pascual, she loses twice against Mauresmo, Capriati and Seles. The thrashing against Serena is just another loss for someone who can't compete any more against ANY of the top-players.
Martina gave it one more try in 2002. And she got a golden opportunity to win another slam (without facing a Williams), and she choked it away which was basically the end of her career.
Yes, Martina HAD painful losses against Venus and Serena, but
a) she could compete in these matches
b) they weren't turning points in her career. The results just don't indicate that.