Williams, a six-time Grand Slam winner, won 7-5, 6-3 at the Foro Italico to claim a place in the last 16 where she will meet either Russian Vera Douchevina or Dally Randriantefy of Madagascar, who play on Wednesday.

Playing in only her fourth tournament since taking eight months off after knee surgery and given a bye into the second round, Williams looked rusty but was happy with her level of fitness.

"I made about 1,000 unforced errors and that pretty much explains it all, but hey, at least I got through it," said the 22-year-old American world number seven, who pulled out ahead of her third round match in Charleston last month when her knee flared up again.

"I think I am playing pretty well and I'm practising very well, I was just a little off the pace in the match.

"I struggled today and hit a lot of loose shots, but I won and I'm very excited. Physically I feel fine, although I haven't played in a while, but all that matters is that I'm out here competing."

Williams, who beat Justine Henin-Hardenne in the final here two years ago, began slugglishly and Sanchez-Lorenzo made the most of her opponent's slow start, breaking in the opening game and holding serve for a 2-0 advantage.

Sanchez-Lorenzo had double break point in the third game, but the 26-year-old failed to take her chances and Williams stormed into a 5-2 lead.

A spirited comeback from Sanchez-Lorenzo drew her level, but she was unable to sustain the momentum and Williams held serve before a superb double-handed backhand smash ensured another break and the first set.

Sanchez-Lorenzo enjoyed an early break in the second set only for Williams to strike back immediately and forge 4-2 ahead.

Williams, whose sister Venus is not taking part here after injuring her ankle in the semi-finals in Berlin last week, held on to her serve and wrapped up the match in one hour and 25 minutes.

In the day's only other second round match, 17-year-old Maria Sharapova knocked out sixth seed Elena Dementieva in an all-Russian clash.

Sharapova said she never looked back after a positive start.

"I was dictating most of the points from the beginning of the match and I started to feel really confident," she said.

"I attacked her serve quite well and when the points got really close, I thought mentally she was a little bit down, and I took advantage of that."

The four other seeded players in first round action all advanced to the next round.

Argentine 12th seed Paola Suarez beat Colombian Fabiola Zuluaga, who was forced to retire due to injury, Israeli 16th seed Anna Smashnova-Pistolesi defeated Spain's Magui Serna and Swiss 11th seed Patty Schnyder overcame German Anca Barna.

Spanish 13th seed Conchita Martinez, a quarter-finalist at the French Open last year and a four-time winner in Rome, ousted Yuliana Fedak of the Ukraine.

Croatian Karolina Sprem, who was 3-0 up in the deciding set of her third round match against Venus Williams in Berlin last week before losing to the American, secured a place in round two with a straight sets victory over Daniela Hantuchova, a quarter-finalist at the 2003 Australian Open.

Venezuelan Maria Vento-Kabchi beat Czech Klara Koukalova and will play American fifth seed Jennifer Capriati in round two, while Italian wildcard Maria Elena Camerin defeated the out-of-form Serbian Jelena Dokic, a Wimbledon semi-finalist in 2000 when she represented Australia.

In the day's late match, another Italian, Flavia Pennetta delighted the home crowd by beating Laura Granville of the United States in straight sets.