Footballers are a competitive bunch. The will to win is rarely ever consigned to the pitch.
So when Manchester United strikers Diego Forlan and Ruud van Nistelrooy faced each other on a tennis court at Nike's HQ in Portland during a 2003 pre-season tour, there was tension.
Sir Alex Ferguson and their United team-mates were watching. Fergie had apparently put money on Forlan winning the pre-training tie-break tussle.
"Everyone wanted to see who was going to win. I had all the pressure," remembers 45-year-old Forlan, who is gearing up for a professional tennis debut in his native Uruguay next month.