Sharapova, who had shoulder surgery in 2008, briefly reach No. 1 last spring after winning the French Open to complete a Career Grand Slam.
Sharapova arrived in Milan to play an exhibition tournament on Saturday with another former No. 1, Ana Ivanovic. They will face Italians Sara Errani and Roberta Vinci in singles and doubles.
"Obviously, the goal is always to shoot for top spot and that's always where you want to be and stay," Sharapova said. "It's one thing to reach the No. 1 ranking, another to keep it."
Errani also had a successful year, improving to No. 6 in the world from No. 45. She reached the final at Ronaldo Garros, where she lost to Sharapova, and the semifinals of the US Open.
Errani won the doubles title at both events with Vinci, and they finished as the top-ranked doubles team.
"It's been an incredible year," Errani said. "Starting from the Australian Open, I had very great results and was consistent, in the singles as well as the doubles. It will be hard to repeat that next year, but I will always try to give my maximum."