US Open key questions: Is Alcaraz fit? Will Sabalenka make history?

The US Open, of all the four slams, is notoriously the hardest to predict the winners of as it falls at a time of year when eight months into the season the players are more likely than not nursing aches and pains with their bodies battered from the grind of the tour. 

It's also played on a surface where more players fancy their chances; many have love-hate relationships with clay and grass but hard courts are their bread and butter.