Will Wimbledon be Step or Stumble on Serena Williams' Historic March?

Novak Djokovic stood on the brink of history after extinguishing Andy Murray's fire in the fifth set of their French Open semi-final . He was one victory away from completing the career grand slam and, if everything went according to plan against Stan Wawrinka in the final, the world No1 would also be a step closer to becoming the first man to sweep all four majors in a calendar year since Rod Laver in 1969. Everything did not go according to plan. ('Federer and Djokovic don't like each other')

Djokovic's tears after losing to Wawrinka do not automatically mean that alarm bells should be blaring inside Serena Williams's head as she attempts to complete the third leg of what many experts expect to be the first calendar slam in women's tennis since a 19-year-old Steffi Graf ruled the sport in 1988 (the German also won gold at the Seoul Olympics to claim the golden slam). Yet they were a reminder that nothing is set in stone in this game, even when something looks like destiny.

Williams is in an enviable but fragile position. The glory on offer over the next three months could spark her game; yet the thought of failure - relatively speaking - could choke her. (Wimbledon 2015: Guide to Key First Round Matches)

She is the overwhelming favourite at Wimbledon and, if she succeeds at SW19 again, the world No1 will need only to win the US Open for the fourth consecutive year to equal Graf's record of 22 grand slam titles in the open era and complete the calendar slam.

Williams has already completed the "Serena slam", holding all four majors simultaneously after winning the Australian Open in 2003 , but this is unlikely to be straightforward. Early defeats by Alize Cornet and Sabine Lisicki at Wimbledon in the past two years are red flags, as was the tangle she got herself into in her win over Lucie Safarova in the French Open final . After letting a 6-3, 4-1, 40-15 lead slip to allow Safarova into what transpired to be a one-sided deciding set, Williams admitted she was affected by the pressure of winning her 20th major. (Federer, Nadal, Murray in Same Half of Wimbledon Draw)

Observers have suggested the US Open will be a formality if Williams wins Wimbledon, given she has won the past three titles in New York. But Graf was almost crushed by the weight of history at Flushing Meadows. "I remember above all the extreme fatigue I felt in New York," Graf told L'Equipe recently. "I was feeling the expectation around me that wasn't mine and that was becoming suffocating. It was terrible." (Roger Federer Still in the Mix at Wimbledon, His Lawn of Dreams)

Jo Durie, the former British No1 who ended her career with a 4-3 winning record over Graf, argues that the greatest impediment to Williams could be, well, Serena Williams. There is a feeling that she is so much stronger than her rivals that she sometimes loses concentration and some of her finals in recent years have been closer than anticipated. (Kevin Curren: 1985 Wimbledon Defeat by Boris Becker a Special Not Bitter Memory)

"Maybe sometimes she's almost trying too hard to do it and gets herself into a bit of frenzy almost on court," Durie says. "Some of the time it's not how her opponents play but how she's battling herself. You still have to play well against her. Cornet beat her going for it, Lisicki did. You have to go at Serena and you have to keep it going because she will come back at you with a vengeance. Mixed up in that is the battle against herself, which you can see very plainly sometimes. She's almost crying with frustration that she's not playing as well as she thinks she ought to."

The differences in the surfaces these days make the calendar slam harder to pull off. The quick switch from the clay of Roland Garros to the grass of Wimbledon has been the downfall of many. Durie points out that three of the slam tournaments were once on grass.

Those who remember Graf reckon that she was a touch more consistent than Williams. There have been times when the American has not been totally enamoured with the game but it should not be forgotten that four days after winning Wimbledon in 2010 she stepped on a piece of glass in a Munich restaurant and suffered a haematoma and a pulmonary embolism that nearly ended not only her career but her life. To recover from that encapsulates her greatness.

Graf was special too. "Nobody could touch Steffi," Durie says. "We had this bit of fun between us that, if you could last an hour against Steffi, you sort of got a cheer as you went back in the locker room. I remember playing her when she was that good and managing to get past the hour mark. She was just so far above everybody else."

Martina Hingis, who almost emulated Graf in 1997, rates Williams as the toughest opponent of her career. "She was the player I least liked to play against," Hingis says. "It was hard to read her serve and she came up with the best when she was down, with danger."

Yet the argument against ranking Williams as the greatest of all time is that her rivals have not pushed her hard enough. There is inconsistency within the top 10. Williams has the measure of Maria Sharapova, the other great champion of this era, while Victoria Azarenka, the former world No1, has slipped down the rankings after struggling with injuries since winning her second Australian Open title in 2013.

"I think the depth is wider today but the top 10 was more stable in our time," Hingis says. "When you look at the top 10, you could really see the difference between the top 10 or five players more than you see today. It's been changing and they haven't been as imposing."

In 1997 Hingis won three majors but lost the French Open final to Iva Majoli. "I was unbeaten, you think you're a little bit unbeatable," Hingis says.

As Djokovic discovered in Paris, the unbeaten run can end quickly.