Williams, the second seed and 2002 champion, spurned eight match points en route to a hard-fought 6-3, 6-7 (5/7), 6-4 victory over Czech Klara Zakopalova.
It was the American´s first claycourt win since the second round here last year and avenged her surprise defeat to the Czech girl in the first round in Marbella in April.
But what looked like a routine win, when she was a set and 5-4 up, turned into a battle of attrition before Williams, the holder of 10 Grand Slam titles, rescued the situation.
"I think I played a very good match and I think Serena will be playing better and better, so it was my best chance to beat her," said a glum Zakopalova.
There was an emotional return to Roland Garros, after a five-year absence, for Australia´s Jelena Dokic.
The 26-year-old, whose career and life were in a tailspin until a magical run to the Australian Open quarter-finals this year, beat Slovenia´s Karolina Sprem 3-6, 6-1, 6-2.
It was her first victory at Roland Garros since she reached the second round in 2003. She played and lost the first round in the following year.
"This is only my sixth tournament since the Australian Open and I feel good. It´s a good sign that I was able to turn it around after losing the first set," said world number 80 Dokic.
"It´s been five years since I played here and only my second Grand Slam in four years. It all feels kind of new."
Dokic, a quarter-finalist in 2002, will now face Russian fourth seed Elena Dementieva for a place in the last 32.
Fifth-seeded Serbian Jelena Jankovic shook off a two-hour rain interruption to reach the second round with a 6-2, 6-3 win over Petra Cetkovska of the Czech Republic.
But as rain fell and temperatures plummeted from the 30 degrees of the first two days, 24-year-old Jankovic was keen to see the green light for the French Tennis Federation´s plan to build a new 14,600-seater arena with a retractable roof.
"It´s always a good idea to have a roof so then we can continue the match and we don´t have to wait around for an hour or more and then come back on court," said Jankovic, who was 4-1 ahead in the first set against her world number 85 opponent when the rain came.
Men´s fourth seed Novak Djokovic opened his campaign with victory when Ecuador´s Nicolas Lapentti, 10 years his senior, was forced to retire with a left ankle injury which needed constant attention in their 78-minute encounter.
Djokovic has been a semi-finalist here for the last two years and has enjoyed an impressive claycourt season with a win in Belgrade as well as runners-up spots in Monte Carlo and Rome.
The 22-year-old also pushed world number one Rafael Nadal all the way in a marathon four-hour semi-final in Madrid where he had three match points before the Spaniard prevailed.
"The conditions were tough today. The wind was powerful and it was hard to judge which side the wind was coming from," said Djokovic.
Argentine fifth seed Juan Martin Del Potro was untroubled in a 6-3, 6-3, 6-1 win over France´s Michael Llodra and will tackle Serbia´s Viktor Troicki for a place in the last 32.
Twelve months ago, Del Potro arrived in Paris as the world 68 and without a title. Now he is the world number five and with five trophies under his belt.
"It was a big change for my game and for my career," said the 20-year-old, before adding a note of caution.
"Now I´m in the top 10, but I´m a long way from being a really good player like Nadal or Federer."
French ninth seed Jo-Wilfried Tsonga and 11th-seeded compatriot Gael Monfils, as well as Spanish 16th seed Tommy Robredo, also made it through.
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