Maria Sharapova, struggling for form after shoulder surgery and languishing 61st in the world rankings, is refusing to be written off as a title contender at this week's Stanford WTA tournament.

"I'm a competitor and played many tournaments and won quite a few," the three-times grand slam winner, who faces Japan's Ai Sugiyama in the first round, told reporters.

"You want to be the winner and if someone tells you otherwise they wouldn't be telling the truth."

However, winning at Stanford will be a tall order for the 22-year-old Russian in a field which includes Serena and Venus Williams and former world number one Jelena Jankovic.

Sharapova won the last of her 19 WTA titles at Amelie Island in April last year. In October, she underwent a shoulder operation and did not return to singles competition until May in Warsaw.

Sharapova produced gutsy displays to reach the French Open quarter-finals but was upset by Argentine Gisela Dulko in the second round at Wimbledon.

"Coming off shoulder surgery I was trying to prepare my arm for the season and it got up to speed, but then I was playing catch-up in tennis department," she said.

"I only had a maximum of five weeks of training before the clay courts. After such a long lay-off, it wasn't enough for me. Even though I pulled through some tough matches at the French Open, physically I didn't have it."

Sharapova said her shoulder was now 100 per cent, but she has abbreviated her service motion and is still trying to adjust the stroke so she can hit her first serve at full power.