Serena Williams is the oldest woman to be ranked number one in the world. (Source: AP)

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, Williams has found herself engaged in a losing battle with the history books.

That Wimbledon final 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, Williams 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 haven’t fully healed for Williams 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 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 the start of her Wimbledon campaign against Swiss qualifier Amra Sadikovic.

The 34-year-old is the oldest woman to be ranked number one in the world and, with off-court interests including the fashion industry and a recent appearance in a video for pop-star Beyonce’s ‘Lemonade’ single, critics have claimed Serena is no longer so focused on her tennis in the twilight of her glittering career.

Given Williams 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,” Williams 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,” Williams 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 Maria Sharapova absent as she appeals against a two-year ban for doping, the main challengers for Serena’s crown should be second ranked Muguruza, former Wimbledon finalist Agnieszka Radwanska and the likes of Kerber and two-time Wimbledon winner Petra Kvitova.

Reflecting the confidence of the chasing pack, Kerber, whose first opponent is British wildcard Laura Robson, said: “Of course Serena’s one of the favourites, but it’s tough to say who will win.

“Everything can happen. I know I can beat everybody.”

Muguruza, who starts against Italy’s Camila Giorgi, is the pick of the bunch after winning her maiden Grand Slam title.

The Venezuela-born 22-year-old’s first round exit in last week’s Mallorca Open was a setback but she is now fully focused on Wimbledon.

“The truth is I’m disappointed, but now I’m just training harder to arrive ready for Wimbledon,” Muguruza said.

“It feels like ages since I last stepped on grass, but it brings me great memories.

“Reaching the final at Wimbledon, that was something so special.”