By John O'Donovan

The French Open is the most unpredictable of the four Grand Slam events on the women’s calendar, with six different winners in the last six competitions.

Indeed, the top seed has not won the event since Justine Henin prevailed in 2007.

It marks the culmination of a gruelling clay court season and many players believe it is the toughest Slam to win because of the mental and physical challenges the red surface throws up over the two weeks.

Serena Williams is the warm favourite for the tournament. The world number one has been in stunning form this season, winning the last four tournaments in a row, three of which have been on clay.

Her victory in Rome last week was her 51st career title and it ensured she remains the oldest ever world number one. En route to those four titles, she has twice beaten world number two Maria Sharapova, as well as Victoria Azarenka, the third best player in the world.

Williams is also currently enjoying a career-best 24-match winning streak (including two Fed Cup matches), and is playing exceptional tennis. Incredibly, she has only suffered three losses in the last year and has won 10 of the 13 tournaments she has played in during that period.

One of these tournaments was her gold medal performance in the London 2012 Olympics where she dropped just 17 games in total and produced arguably the greatest display of tennis ever seen from a female player.

With such an array of stats, one may wonder why this tournament should not be a foregone conclusion.

For starters, Williams has always struggled in the French Open. She has only won the title once (in 2002 on her way to completing the ‘Serena Slam’) as opposed to every other Grand Slam event which she has won at least four times.

Indeed, she has not even reached the last four in Roland Garros since 2004.

In 2012 she was in a similarly good vein of form coming into the competition having not lost on clay for almost two years (she missed the 2011 French Open through injury).

She then proceeded to lose in the first round of a Grand Slam for the first time in her illustrious career, defeated by Virginie Razzano, ranked 111.

Williams was also red hot favourite to win the Australian Open in January of this year but suffered a shock quarter-final defeat to Sloane Stephens. Her patchy history in the French Open is what her opponents will cling to in the face of her awesome displays of tennis this year.

If Serena does slip up, the defending champion Sharapova will be hoping to take advantage.

She had to withdraw from the Rome Masters last week at the quarter-final stage due to a virus and a week before that she was overwhelmed by Williams in the final of the Madrid Masters.

Should she have to face the American she has very little chance of victory as she has not beaten her since 2004 and her career record stands at just two wins from 15 encounters. If she does not have to play Williams however, she has a great chance of winning the tournament.

She has won two competitions this year including one on indoor clay. She has also developed a liking for clay, a surface she once described as making her feel “like a cow on ice.”

Sharapova has tremendous groundstrokes and is a great defender although her serve is the one area of her game which is liable to let her down in crucial matches.

Victoria Azarenka, the third seed and the reigning Australian Open champion, is another player who will consider herself a contender in Paris even if others do not.

She lost to Williams last Sunday in the final in Rome and has never gone further than the quarter- finals at Roland Garros.

The world number three has not been in good form since winning the Australian Open and has suffered from niggling injuries throughout the season.

She won the Qatar Open in January but last week’s final was the first decider she has reached since then and has won only won a single clay court title in her career. The Belarusian will need to improve her first serve percentage and cut down on the errors in her return game to have any chance of victory.

Li Na is a more likely challenger than Azarenka. The Chinese player is the best mover on the women’s circuit. She won the title in 2011 and the very best of her often comes out on clay.

She is a tenacious flat hitter of the ball and was in top form over the early stages of the season. She reached the final of the Australian Open before missing two months through injury. She has come back well and got to the final of the Porsche Grand Prix in Stuttgart in late April before losing to Sharapova.

Li is now coached by Carlos Rodriguez, who took Justine Henin to four French Open titles. Rodriguez has the tactical knowledge that should make her a more complete player than she was when she won the French Open in 2011.

Another real contender is last year’s finalist Sara Errani, who also plays her best tennis on clay. The Italian is one of the best at control and variety of play on the surface. She has had very consistent results in Grand Slam events in recent years although she has a poor record against top ten players.

Meanwhile, Agnieszka Radwanska and Petra Kvitova are ranked fourth and seventh in the world respectively and are players who could make the semi-final or even final but it is difficult to see either winning the tournament.

Samantha Stosur, runner-up in 2010, is another player who could go far. She has had a frustrating time with injuries this season although she appears to be coming back into form, beating Kvitova last week before narrowly losing in three sets to Azarenka.

German number one Angelique Kerber also appears to be coming back to her best at the right time of the season. She has great power in her game and is a tricky opponent and should reach the quarter-finals at least. Anastasia Pavlyuchenkova is an up and coming promising player who has won the Portugal Open on clay and the Monterrey Open in Mexico already this year. The 21-year-old will not win the tournament but may cause an upset and is one to watch.

While the list of possible contenders is lengthy given the unpredictable nature of the competition, Serena Williams is still the safest bet to win her second French Open, 11 years after winning it for the first time as a 20-year-old.

Selection: Serena Williams