Loit gained the first break of the final set for 3-2, but Williams immediately broke back.
The French player saved two match points while trailing 5-4 with a forehand winner and a drop shot, but then couldn't hold serve despite leading 40-30 in the last game.
The Australian Open has been troublesome in the past for both Williams sisters. Serena never has advanced past the quarterfinals. Venus was a semifinalist once, in 2001.
On the men's side, No. 3 Marat Safin, last year's runner-up, beat Dutch player Raemon Sluiter 6-4, 4-6, 6-3, 6-4, and Gustavo Kuerten, a three-time French Open champion who now is seeded 30th, beat Morocco's Hicham Arazi 6-4, 7-6 (8), 6-3.
No. 6 Roger Federer needed nearly three hours to beat Brazil's Flavio Saretta 7-6 (4), 7-5, 6-3, and No. 7 Jiri Novak defeated American Vincent Spadea 6-2, 2-6, 6-2, 7-6 (3).
Wimbledon runner-up David Nalbandian advanced when France's Jerome Golmard quit after three sets with a back injury.
Nalbandian led 3-6, 6-3, 6-1 before the withdrawal by Golmard, who also hurt his back in a tournament at the end of last year in India.
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