Williams came from a break down to win the opening set

Australian Open champion Williams was in action for the first time since withdrawing from last month's Fed Cup tie against Belgium.

But the eighth seed struggled before eventually going through 6-4 6-1.

Seeded trio Patty Schnyder, Katarina Srebotnik and Tathiana Garbin also advanced in straight sets.

Schnyder, the 13rd seed, who was a Rome finalist two years ago, beat Vasilis Bardina 6-1 6-2.

I don't think I warmed up enough inside Serena Williams

Srebotnik defeated Martina Mueller 6-4 6-2 and Garbin eliminated fellow Italian Flavia Pennetta 6-3 7-6 (7-3).

Williams had trouble gaining her footing on the red clay and was broken early in the first set.

But she got into her rhythm and hit several powerful backhands to break back and level the set at 4-4 and then applied more pressure on the Krajicek serve to break again and win the set.

She raced into a 4-1 lead in the second set and was able to ease to victory.

"I came out flat. I don't think I warmed up enough inside and she was playing really well," Williams said.

World number seven and eight Nicole Vaidisova and Ana Ivanovic both withdrew from the tournament on Tuesday with injuries.

Vaidisova has a right wrist injury while Ivanovic is struggling with an ankle injury after winning last week's German Open.