Williams is one Wimbledon title away from completing what she calls the Serena Slam for the second time in her career - having followed up her 2014 U.S. Open success with victories at this year's Australian and French Opens - but must overcome her recent lean spell on grass.

After winning the last of her five Wimbledon titles in 2012, the American world number one has fallen before the quarter-finals in her last two outings at the grasscourt major.

"I think Serena's got a great shot," Evert, winner of 18 grand slam singles, said. "To me, her game is better suited to the grass courts than it is to the clay. She struggled through the French, remarkably winning it."

Three-times Wimbledon champion McEnroe said it was there for the taking for Williams, who is bidding to win her 21st grand slam singles title.

"Serena's ... chasing history in terms of her grand slam titles," McEnroe said. "Even though she's still playing arguably her best tennis, you have to wonder at a certain point how long that can keep up."

The American commentators agreed that the lapses of concentration that hampered her at Roland Garros could prove more costly at the All England Club.

"When she is at her best she is better than anybody else," Evert said. "But at the same time we've seen some hiccups and we've seen some drama, like at the French Open. She can't afford to have any more drama like at the French Open."

Williams battled flu during the second week of her run at Roland Garros, where she survived five three-set battles to win the title, beating Lucie Safarova in the final.

"On grass, a couple bigger hitters ... (could) win more points on the serve than they would on a clay court or a hard court," McEnroe said.

"They're not as easily attackable because of the nature of the grass being a little bit more difficult to gauge where the ball's going to be..."

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