Is it the stamina needed to last a match or the power to slam a serve down at 100mph?

Tim Henman's former fitness coach, Kieron Vorster, believes it is a combination of the two which helps most.

He says: "Tennis is a power-based sport which uses energy between 0-10 seconds.

"So in the ideal tennis athlete you want a lot of power endurance."

Power endurance is the ability to repeat short bursts of power over a long period of time.

And with tennis matches taking anything up to five hours the endurance is the key factor.

Roddick's game is all about power

Kieron says: "In training, you want to do a lot of work which takes between five and ten seconds, giving yourself 25 seconds rest because that's how long it is between a point.

"Then you want to be able to repeat that over a long period of time so it is specific to tennis."

On some surfaces such as clay, a ten-second point may seem conservative but Kieron disagrees.

He says: "A stat taken in the 2001 semi-final of the French Open between Gustavo Kuerten and Juan Carlos Ferrero proved there's a myth that points are longer on clay.

"The average point time in that match was just over seven seconds."

So no matter the surface, power endurance is your key to being fit for tennis.

Comparing Tim and Andy

Kieron worked with Henman between 1999 and 2002 but most would agree that Tim is far from being the most powerful player on Tour.

So why didn't Kieron work on Tim's power endurance when they were together?

Kieron says: "Players have different physiques and for the game that Tim plays he needs to be slight.

"What you have to be careful of is when you're building up a tennis player you're not building up non-essential muscle.

Henman's frame is slight to help his agility

"The way Tim plays as a serve-volleyer means that he's got to be quick into the net or back for a smash.

"He's got to be agile, he's got to be fast and if he's lacking in any of those areas then it will hinder his game.

"So when I worked with him the key things we worked on were his agility, his speed and at the same time his power endurance.

"But just because he is slight it doesn't mean he lacks power. There's a lot of athletes like that.

"On the other hand Andy Roddick is a prime example of somebody who has the raw power.

"So the area he needs to work on is repetitive endurance so he can sustain that power.

"The counter-argument to that is looking at the way he plays.

"The points are over in six or seven seconds. He's using his power to finish the point quickly so he's not out there for as long a time."