Serena Williams launched her clay-court campaign in Rome playing for her first title since Cincinnati last August—and ramp up a run to defend the top spot.

The world No. 1 beat Garbine Muguruza to win Wimbledon last July.

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The fourth-ranked Muguruza, who gave Williams one of the worst Grand Slam losses of her career at the 2014 French Open, says the 34-year-old American's decline is inevitable.

"I think, yeah, a little bit it's changing," Muguruza said about Serena's stronghold on the top spot. "Obviously I think it's normal. At some point other players are going to get closer to that spot there."

Williams holds a 2,500-point rankings lead over world No. 2 Angelique Kerber, but the 21-time Grand Slam champion will be defending 4,000 ranking points with her Roland Garros and Wimbledon titles on the line in the coming weeks.

Given the numbers and Serena's advancing age, Muguruza says "it's a matter of time" before Williams is displaced from the top spot.

"I think it's just a matter of time, honestly," Muguruza told the media in Rome. "There is a lot of players out there fighting for it, and Serena eventually is going to, you know, go a little bit down because she's like forever there. So we'll see."

Asked if she believes she can eventually rise to No. 1, Muguruza said: "I think I need to improve so many stuff to be there, but, well, I'm trying. I think I can make it."

Photo credit: Roland Garros