It's the vulnerability that makes the Serena Williams narrative so compelling this summer.

Even more compelling than during last year's US Open when the same player was presented - before she ran into Roberta Vinci in the semi-finals - as Serena The Invincible. 

You might say that Williams' vulnerabilities have never been as exposed to the wider tennis world, and to the public, as they are right now. In part, that's thanks to an all-access documentary, recently aired, that followed her last season as she attempted to become the first player since Graf in 1988 to accomplish the calendar-year Grand Slam.

Undefeated in Melbourne, Paris and London, she came unstuck against Vinci in one of the greatest upsets in tennis history, which left her "in a dark hole". It's a different landscape for Williams this season. After defeats in the Australian Open and Roland Garros finals this season, there had been some concern about when - or if - she would achieve parity with Graf on 22 majors. 

That concern had evidently spread to Williams herself. After equalling Graf's tally on Centre Court, she declared that "enjoying the moment" was the sole entry on her agenda, given that the pressure of chasing No.22 had proved so debilitating. 

For the best, though, the pursuit of history is unending. The time to enjoy the moment is over, and Williams will once again be swinging for sporting immortality at Flushing Meadows. She will never again play at the US Open under the same weight of expectation and pressure as she did last summer, but questions persist, particularly in light of Rio.

And yet, all this doubt surrounding Williams could end up working to her benefit - it certainly did at Wimbledon.