Andy Murray claims he and brother Jamie did not play much tennis together when the pair were young - they were too busy playing other sports, with Jamie often the victor.
Jamie and doubles partner Bruno Soares secured the year-end world number one spot after Pierre-Hugues Herbert and Nicolas Mahut lost their final round-robin match at the ATP World Tour Finals.
That achievement opens up the possibility of a family double for the Murrays, with Andy currently top of the men's rankings and into the semi-finals of the singles draw in London.
With all that on-court success, one might imagine the Murray brothers - whose route to the top saw them living apart during their teens - spending hours knocking a ball back and forth across a net as children.
However, while Andy accepts the existence of sibling rivalry, he says it often manifested itself in sports other than the one in which they have achieved such success.
"Between 12 and then 17 and 18, we didn't spend loads of time together," he said. "But before then, we were on the court together. Pretty much every time we went to practice or play tennis, it was together.
"But we didn't play loads of tennis. We played probably four hours a week up to that point. It wasn't loads.
"But we did play golf together. We played squash together, table tennis. We were always competing against each other from a young age.
"When we were really young, Jamie would have won most things we did. He was 15 months older, so he was bigger and stronger and better than me at most things. He was smarter than me.
"Then as we started to get older and physically were on more of a level playing field, we were pretty close really at most things.
"Jamie is a really good golfer. He was better at that. Football, probably me. Then squash and table tennis, more of the racquet sports, was pretty close between the two of us really.
"Now we obviously don't [play against each other]. I think we're probably each other's biggest fans.
"It's really special to get to watch what he's achieved in the biggest competitions in the sport.
"Neither of us ever would have expected this when we were growing up. [We] need to try and enjoy it."
Andy faces Milos Raonic in his semi-final clash on Saturday following his 6-4 6-2 victory over Stan Wawrinka, with Jamie and Soares also in last-four action.