The top-ranked American fell ill after winning in Melbourne and wasn't able to practice until Monday.
"I was in bed. I was really sick until I came here,'' Williams said. "It was like a week. I couldn't do anything.''
Second-seeded Jelena Jankovic, third-seeded Elena Dementieva, fourth-seeded Agnieszka Radwanska, sixth-seeded Patty Schnyder and eighth-seeded Amelie Mauresmo also won on a perfect day for seeds in the indoor event.
Radwanska and French qualifier Emilie Loit were the first players to move into the quarterfinals.
Radwanska played until 12:46 a.m. to outlast Alisa Kleybanova of Russia 6-7 (5), 6-4, 6-2, while Loit rallied past Kaia Kanepi of Estonia 2-6, 6-3, 6-1.
Williams saved two break points on her first service game before winning six straight games to take the first set on an unforced error from Benesova.
"I wasn't great. I was so glad to win,'' Williams said. "As the tournament goes on, I'll do a little better.''
The 34th-ranked Czech fought back in the second set, but Williams won the match on Benesova's serve with a backhand winner down the line.
"She raised her game and I made some key errors on some key points (in the second set),'' said Williams, who will next play Croatian qualifier Karolina Sprem.
Williams, who recently bought an apartment in Paris, playfully answered some questions from the local press in French.
"I feel so-so. I'm tired right now. I'd like to sleep,'' she said in French.
Jankovic defeated Francesca Schiavone of Italy 7-5, 7-6 (2), winning the first set on a double-fault before saving a set point at 5-4 to force a tiebreaker.
"It's a relief to finish in two sets,'' the Serb said. "After the first set and 3-0, I really lost my energy and my game went down. I let my opponent come back into the match. Until then, I was completely in control.
"I'm lucky that I was somehow able to get through the second set. As a top player, when you have everything under control, you have to finish your opponent off.''
Schnyder beat Alona Bondarenko of Ukraine 6-4, 7-6 (3) while Mauresmo avenged her Fed Cup loss last weekend to Sara Errani by routing the Italian 6-2, 6-0.
Dementieva ousted Aravane Rezai of France 6-4, 6-3.
"I had some problems with my serve,'' said the fourth-ranked Russian, who landed only 47 percent of her first serves in the first set. "It didn't work well today, especially in the first set. Aravane has a very good return and she put a lot of pressure on my second serve today.''
In the second round, Jankovic will face Li Na of China and Schnyder will meet French wild card Nathalie Dechy.
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