The self-designed cheeky little number she wore as she brushed aside Spaniard Arantxa Parra Santonja 6-2 6-4 in the second round drew more questions than her game.
Williams wore the same black lace seethrough skirted bodice that exposed her skin-coloured underwear she had worn against Patty Schnyder in the first round.
But the American, who also caused controversy with her outfit at the Australian Open, denied the dress was deliberately provocative.
She said: "That was never the objective. It all started in Australia where I had a dress with really long slits in the front. It was about wearing a dress that looks like you have these slits with bareness.
"This design has nothing to do with the rear. It just so happens that I have a very well developed one.
"The illusion of just having bare skin is definitely, for me, a lot more beautiful."
She was asked one question about her victory over Santonja and Williams said: "On the bigger points, especially on my serve, I played those really well, and I think that was key."
Williams now faces 26th seed Dominika Cibulkova of Slovakia who avoided an upset after coming from a set down to crush Varvara Lepchenko from the USA 4-6 6-2 6-0.
On a rain-hit fourth day at Roland Garros 14th seed Flavia Pennetta followed up her victory over Britain's Anne Keothavong with a 6-1 6-1 success against fellow Italian Roberta Vinci.
Russian Nadia Petrova (19) cruised past Hungary's Agnes Szavay 6-1 6-2, while Romainan Alexandra Dulgheru (31) defeated Timea Bacsinszky of Switzerland 6-4 6-2.
