LONDON - Rafael Nadal and Novak Djokovic head to Wimbledon fearing the title may be beyond them as the season's third Grand Slam tournament shapes up to be the most open in a decade.

World number one Nadal, fresh from a record ninth French Open title, was Wimbledon champion in 2008 and 2010 and runner-up in 2006, 2007 and 2010. But his last two visits have been humiliating disasters. The Spaniard suffered his first ever opening round exit at a Grand Slam in 2013 to Belgian journeyman Steve Darcis who was ranked 135 at the time and has not won a main tour match since. Twelve months earlier, the world's 100th best player, big-hitting Lukas Rosol put him out in the second round, a defeat which precipitated a seven-month absence from the sport for the man from Mallorca.

The 28-year-old hinted at another Wimbledon letdown in the immediate aftermath of his triumph over Djokovic in the French Open final two weeks ago where he claimed his 14th major. "I am healthy, that's the most important thing. I hope my knee will have a positive feeling on grass because I felt my knee was better last year on the other surfaces," said Nadal, who has been seeded number two for Wimbledon. "Grass is always a little bit harder for me after injury. I played Wimbledon in 2012 with my knee injury and I never played another match after. Last year I tried but I was not ready enough to compete. Let's see how are my feelings this year."

Those feelings would not have been boosted by an opening exit on the grass at Halle last week, a straights sets loss to German world number 85, Dustin Brown. "The match was negative in all ways," lamented Nadal. World number two Djokovic, who won his only Wimbledon title in 2011 and was runner-up to Andy Murray in 2013, has not played a grasscourt warm-up event since 2010. The 27-year-old Serb, the top seed for Wimbledon, won the last of his six majors at the Australian Open in 2013. But his latest thwarted attempt to win a first French Open and become just the eighth man to complete a career Grand Slam represented his seventh defeat in 13 finals at the majors. Even more worryingly, Djokovic has now lost five of his last six Grand Slam finals.

The coaching role of three-time Wimbledon champion Boris Becker has never looked so crucial. Meanwhile, Murray, having become the first British man since Fred Perry in 1936 to win Wimbledon when he trounced Djokovic in straight sets last year, goes into the tournament with his form also giving cause for concern. The world number five, who recently hired Amelie Mauresmo as coach, hasn't reached another final since and lost in the third round at Queen's Club last week to 35-year-old Czech, Radek Stepanek.

"The difference between this year and last year is that I've played a lot of matches on clay in the last couple of weeks this time," said Murray, who made the semi-finals at the French Open, losing to Nadal. "Last year I had about a week to 10 days' preparation on grass before Queen's." Murray has been seeded three for Wimbledon, above seven-time champion Roger Federer who may have most reasons for being confident about an eighth title having captured the Halle grasscourt trophy for the seventh time at the weekend.

The 32-year-old, who won the last of his 17 majors at Wimbledon in 2012, hopes his success in Germany is a sign of good things to come in south-west London. "In the past, when I have played well at Halle I have usually played well at Wimbledon," said the Swiss star. "They have been two of my most successful tournaments, so I hope that this title will bring me luck again." Last year, Federer was knocked out in the second round by Ukraine's world number 116, Sergei Stakhovsky, ending his run of 36 straight Grand Slam quarter-finals.

At the French Open this year, Federer slumped to a fourth round loss to Latvia's Ernests Gulbis -- his earliest Paris exit since 2004. Should he triumph in London Federer would succeed Arthur Ashe as the oldest men's champion and break free of his seven-title tie with Pete Sampras.

Meanwhile, Serena Williams has set her sights on erasing the bitter taste of a disappointing 2014 campaign by re-establishing her supremacy at Wimbledon.

Williams heads to the All England Club mired in something of a slump following earlier than expected exits from both the Australian and French Opens. After suffering a shock fourth round loss to Sabine Lisicki at Wimbledon 12 months ago, Williams embarked on an extraordinary run that saw her finish 2013 by winning 32 of her 33 matches, a sequence that brought her a 17th Grand Slam title at the US Open. Since then, she has lost her momentum and although she has won titles in Brisbane, Miami and Rome, she has also suffered underwhelming losses to the likes of Petra Kvitova, Jana Cepelova, Ana Ivanovic and Alize Cornet.

Serena's struggles have prompted some to suggest she is finally in decline at age 32. But those chastening defeats have only strengthened her desire to make amends in south-west London and few would bet against the ferociously competitive American adding a sixth Wimbledon singles title to her glittering CV by the time the women's tournament comes to a conclusion on July 5. She starts her 15th appearance at the grasscourt Grand Slam as the world number one and top seed, and should she move one title ahead of her sister and fellow five-time Wimbledon winner Venus, Serena might well label the defeat against Garbine Muguruza at Roland Garros as the turning point.

That second-round exit was a hammer blow to Serena, who lives in Paris for part of the year and works with French coach Patrick Mouratoglou, and, before heading home to Miami, she vowed to step up her practice in a bid to avoid a repeat at Wimbledon, which gets underway on Monday. "I'm going to go home and work five times as hard to make sure I never lose again," she said. "If I couldn't play better I would be even more disappointed. But I know I can, so I know I have something to look forward to."

With Marion Bartoli now retired following her maiden Grand Slam triumph at Wimbledon 12 months ago, this will be the first time since Steffi Graf in 1997 that the reigning women's champion hasn't returned to defend her crown. In Bartoli's absence, it is the fired-up Serena who most expect to carry off the Venus Rosewater dish awarded to the female champion and the 29-year-old Frenchwoman agrees with that verdict. "Grass is a very specific surface and you have to have some experience on that," Bartoli said.

"I very highly favour Serena for it. Not only as she has won so many times but it is a surface that suits her game so beautifully. "When you saw her winning at the Olympic Games, the way she played, the way she beat everyone. "She destroyed Maria Sharapova in the final and I don't think all those young players will be able to challenge her." Sharapova, fresh from winning the French Open for the second time, is seen as the biggest threat to Serena.

But the Russian, who is seeded five, has lost her last 15 matches against Williams, and even the memory of her famous Wimbledon final win over the American as a teenager 10 years ago may not be enough to slay those demons. Other potential contenders include China's Li Na, who started the year in fine style by winning the Australian Open, although the 32-year-old second seed has never been past the quarter-finals at Wimbledon.

Romania's Simona Halep, who has won more titles in the last 12 months than any woman other than Serena, is the third seed after an impressive surge up the rankings capped by her run to the French Open final. Poland's Agnieszka Radwanska, beaten by Serena in the 2012 Wimbledon final, is seeded fourth, while former world number one Ivanovic looks capable of challenging after winning on grass in Birmingham last week.