Consider the monster serve. Too late. You've already been aced.
Of all the mind-boggling concepts in sport, from the bicycle kick to running 100 metres in 9.58 seconds, the big tennis serve may be the most daunting.
A player perhaps 24 metres away is powering the ball at you at speeds touching 225kph, if he is one of the big hitters, and the ball is upon you in just over one second. Getting a racket on the ball seems exceptional. Keeping it in play seems almost unbelievable.
• Dokic is back in the reckoning
Thus, players around the world probably sighed when the record for "fastest serve" was broken again, by Ivo Karlovic on Saturday.
The towering Croat pounded a first serve during a Davis Cup match against Germany at 251kph, eclipsing Andy Roddick's mark of 249kph in 2004.
"I noticed that while serving, saw it on the screen," said Karlovic, who is the tallest player on the ATP circuit at 6ft 9ins. "It is really great."
Great? Seems debatable.
How does a player return a 251kph serve? Close his eyes and take a swing where the ball might be? Stand still and hope it goes long or wide? It brings to mind tales of baseball hitters matched against the fastest pitchers. One such player turned to the umpire to argue a call of "strike" by saying: "It sounded low to me."
Luckily for tennis, most big hitters have flaws. Karlovic and Ivan Dodig lost to Christopher Kas and Philipp Petzschner, making the Davis Cup quarter-finals safe from Karlovic's rockets.