As if we didn’t know this already, caffeine is one of the best ergogenic aids for both recreational and professional athletes, and Serena Williams' apparent random espresso request in the middle of her Hopman Cup match against Flavia Pennetta, seemed to further substantiate this.
Not only did the world number one go ahead and consume a shot of the good stuff between games (whilst 5 games down), she then went on to win the match in a domineering fashion.
Caffeine can stimulate you and maintain focus, but it also has the added benefit of reducing perceived exertion i.e. you don’t find physical activity as strenuous as you might have done before caffeine consumption. If you feel like you’re struggling during exercise, try adopting Serena’s tactics, or alternatively opt for a caffeine supplement of some sort… frankly I’m surprised more athletes don’t do this.