Serena Williams and her sister Venus are the greatest enigma in the women's game, according to former world number one Chris Evert.
Between them, the American siblings have clinched just four grand slam singles titles out of 17 in the last four years but Evert views them as the most likely winners in the big events when on top form.
"The question mark is always the Williams because they come in and out of the game," American Evert, 53, told Reuters in an interview.
"The big question is: `Are the Williams sisters healthy?'. They are injured or they are not focused or whatever. If Serena is healthy and she is focused, she is still the player to beat."
Tennis great Evert, who retired in 1989 after piling up 18 grand slam singles crowns, still monitors the game at the highest level.
"I watch the grand slams and I keep in touch with how everybody is doing," she said during last week's Pebble Beach National Pro-Am golf tournament where her fiance Greg Norman, also a former world number one, was competing.
Asked which women's tennis players particularly impressed her apart from the Williams sisters, Evert replied: "Maria Sharapova is an excellent pressure player and she reacts perhaps the best of all the current players.
"She is mentally focused but maybe not as quick as the others. But that is why she won this year's Australian Open, she was really focused.
"(Justine) Henin is very consistent and the two girls from Serbia, Ana Ivanovic and Jelena Jankovic, are doing great and adding a lot of colour to the game. But it really hasn't changed that much at the top, to me, in the last five years."
Evert had no hesitation when asked what gave her the most pride during her own glittering career which featured 157 singles titles and eight doubles crowns.
"What gives me the most pleasure is how consistent I was," she said. "I was in the top four, I think, for 18 years and I'm also very proud of my clay court record, having won 125 matches in a row.
"Just the consistency and I think I was 90 per cent in winning matches. I am very proud of my mental capacity and my focus."
Dubbed the "Ice Maiden" because of her calm but steely on-court demeanour, Evert launched her clay court run of 125 consecutive victories in August 1973.
Her record spell ended on May 12, 1979, when compatriot Tracy Austin beat her in the semi-finals of the Italian Open.
Her career win-loss record in singles matches was 1,309-146: a 90 per cent success rate which is unprecedented in both the men's and women's games.
Although current men's world number one Roger Federer is yet to win a French Open title to boost his overall grand slam tally of 12, Evert brackets him among the three best players in history.
"Pete Sampras is already up there, along with Rod Laver, and I think you would have to put Roger Federer in there as well," she said.
"Those three would have to be the best ever and Pete never got really close to winning the French and Roger has so I would put Roger up there, definitely," added the American referring to Federer's two final appearances at Roland Garros.
Twice married Evert, who announced her engagement to Australian Norman two months ago, remains closely involved with tennis through her long established academy in Boca Raton, Florida.
"It is a full-time thing with dormitories and schools," said Evert, who also has tennis centres being built in South Africa and Dubai. "It's only seven minutes' away from my house and I usually go there to coach three or four mornings a week.
"But the whole thing is a juggling act, trying to balance everything when I also want to spend a lot of time with my three children - and Greg."