By Martyn Herman

LONDON (Reuters) - The peak remains tantalizingly close for Serena Williams but negotiating the final incline to a record-equaling 22nd grand slam singles title is proving the trickiest of all.

American veteran Williams gets another chance to match Steffi Graf's professional era benchmark at Wimbledon when the championships start next week, having proved uncharacteristically vulnerable when it matters.

Had it not been for defeat by Germany's Angelique Kerber in the Australian Open final and Garbine Muguruza at the same stage in Paris, 34-year-old Williams would have already passed Graf and would be set to tie Margaret Court's all-time record of 24.

Instead, it has all become a bit of a struggle.

Add in her U.S. Open semi-final defeat by Roberta Vinci last year when a calendar slam seemed in her pocket, there are doubts about her finishing skills -- and her nerve.

"Something is holding her back, and it could be nerves," 18-times grand slam champion Chris Evert said.

"Especially against Kerber and against Muguruza, she wasn't able to dig herself out of the hole like she has in past years.

"She's been famous for finding that next gear, that next level, when she's down."

Three-times Wimbledon champion John McEnroe tips Williams to win a seventh All England Club crown, but says self-imposed pressure could be mounting.

"It's not easy to try to do what she's doing, to make history at this stage," he said.

"There's not that many people that wouldn't pick her here. If she's playing well, she'll win the tournament."

Of the challengers, they come in two distinct categories.

The power hitters such as Spain's Muguruza, twice champion Petra Kvitova, former world number one and rising American Madison Keys. Sadly Victoria Azarenka withdrew with injury.

Then there are the scramblers like world number three Agnieszka Radwanska, Kerber and Simona Halep.

World number two Muguruza pushed her hard in last year's final and overpowered her in the French Open final with a fearless display of attacking tennis.

Whether or not the 22-year-old can make the quick switch from clay to grass and deal with the new-found expectation is questionable though, according to Evert.

"I'm curious to see how she does that," the American, who will be commentating for ESPN, said in a phone conference.

On her day Kvitova can be unplayable, especially on grass, although the Czech, winner in 2011 and 2014, is still prone to puzzling off-days and she played down her chances after losing to Johanna Konta at Eastbourne.

"I'm not personally seeing chances, unfortunately. I will be very happy if I can show up in the second week," Kvitova, who will be seeded 10th, said.

Keys, seeded ninth, reached the quarter-finals last year and warmed up for Wimbledon by winning the Birmingham title on grass. She also won the Eastbourne title in 2014.

Evert believes the 21-year-old is now ready to mount a serious Wimbledon challenge.

"This girl can play on grass. Her serve I think matches Serena's," Evert said. "I think she's going to win some majors. I don't have any doubt in my mind."

(Editing by Pritha Sarkar)