LONDON • Tennis' world No. 1 Serena Williams finds herself under siege from revitalised rivals and an army of doubters, as the defending champion starts her bid for a seventh Wimbledon title and a record-equalling 22nd Grand Slam crown.

Since she walked off Wimbledon's Centre Court cradling the Venus Rosewater Dish awarded to the women's champion nearly 12 months ago, she has found herself engaged in a losing battle with the history books.

That victory over Garbine Muguruza meant Williams had won all three of the year's major titles, putting her within touching distance of becoming the first woman to secure a calendar Grand Slam since Steffi Graf in 1988.

But her historic bid came to stunning end in the US Open semi-finals, when she was beaten by 300-1 outsider Roberta Vinci.

In the aftermath of that chastening September day in New York, the 34-year-old has appeared a more vulnerable figure.

For so long, Williams' power game and competitive instincts intimidated opponents into submission, but this year she has won only one of her five tournaments, in Rome in May, while enduring a pair of shock defeats in the Australian and French Open finals.

There is a growing sense the emotional scars from the US Open have not fully healed for her and Germany's Angelique Kerber took advantage to shock her in Melbourne in January, while Spain's Muguruza avenged her Wimbledon loss by beating Serena in Paris earlier this month.

Falling at the final hurdle twice this year has left Williams still stuck on 21 Grand Slam singles titles - one short of Graf's Open era record of 22 and three behind the all-time record of 24 set by Margaret Court - ahead of Wimbledon, which gets under way tomorrow.

Given that she compiled a remarkable 53-3 match record in 2015, even she had to admit 2016 has been a disappointment by her sky-high standards.

"Not as great as I want it to be," she said when asked to assess her year so far.

"I could do better. But honestly, that's how I felt about 2015."

In the circumstances, Williams will be relieved to feel grass under her feet as she returns to the venue where she won the first of her six Wimbledon titles in 2002.

"I've had people put me down because I didn't look like them," she said in a recent documentary.

"I've had people look past me because of the colour of my skin. I've had people overlook me because I was a woman. I'm still going."

With five-time Grand Slam champion Maria Sharapova absent as she appeals against a two-year ban for doping, the main challengers for Williams' crown would likely be second-ranked Muguruza, former Wimbledon finalist Agnieszka Radwanska and Kerber.

Muguruza, who last year became the first Spanish woman to reach the Wimbledon final since 1994, is the pick of the bunch after winning her maiden Grand Slam title in such composed and combative fashion at Roland Garros.

AGENCE FRANCE-PRESSE