(Reuters) - An aching Roger Federer made up for lost time on Wednesday as he saw off Carlos Berlocq of Argentina 6-3 6-4 to reach the third round of the Rome Masters.

The Swiss only decided to play after his morning practice session, carrying a few aches and pains after winning the Madrid Masters title last weekend.

Adjusting from the slick, slippery blue clay of Madrid to the far slower red clay of the Foro Italico with ease, he admitted he had been unsure what to do up to the very last minute.

""I wanted to see how I felt and make sure that I didn't have anything major because we're ahead of a long summer," he told a news conference.

""If this was the last tournament of the season it would be no problem. "But as this is the beginning of many tournaments in a row, potentially a lot of five-setters in a row, I need to make sure I make good decisions, so I go round by round.

"I'm happy that I felt OK out there today."

A good performance this week from Federer, who now plays former world number one Juan Carlos Ferrero, would help him to close the gap on Novak Djokovic in top spot.

""I might be world number three again next week but that doesn't matter for Rafa (Nadal, the number three) and myself," he said.""It's interesting times right now and I know there's going to be a lot of talk about the rankings.

""What I know is that I'll have a shot until the U.S. Open, whatever shape or form with the rankings, and we'll see how it's going to play. Right now I'm just happy I'm healthy."

Tomas Berdych, beaten by Federer in the Madrid final, eased past Lukas Kubot 6-4 6-1 to reach round three, while Juan Martin Del Potro also advanced.

In the women's event, Serena Williams made it two wins in the space of 18 hours as she battled back to beat Russia's Nadia Petrova 4-6 6-2 6-3.

""It was very difficult for me today," Williams told a delighted crowd in Italian. "I hope that I can play better from here."

There were defeats, though, for world number three Agnieszka Radwanska and former world number one Caroline Wozniacki.

Radwanska was ousted 6-4 4-6 6-1 by Petra Cetkovska of the Czech Republic while Wozniacki, suffering from sinus problems, quit when trailing 6-4 4-0 to Anabel Medina Garrigues of Spain.

Nadal was due to begin his title campaign later on Wednesday against Florian Mayer of Germanywhile women's world number one Victoria Azarenka was scheduled to play Shahar Peer in the late match.

(Editing by Pritha Sarkar)