It’s a heated debate which has been disputed online this week as stubbornly as a decisive line call on match point at Wimbledon - are tennis balls green or yellow?

Twitter polls attracting thousands of votes on the surprising issue failed to establish a definitive answer, before Roger Federer finally settled the debate with the same effortless authority he displays on the court.

“They’re yellow, right?” responded the smiling Swiss tennis great to the unusual question from an inquisitive father ahead of the Miami Open.