Remember when professional players' tennis coaches did exactly what it said on the tin?
Coaches never used to be legends of the sport with a string of Grand Slams to their name. They never used to be celebrities in their own right. They were very ordinary ex-players (normally male) who happened to be well versed in the tactics and technique of the sport. They were excellent at mundane jobs such as feeding balls, peeling bananas and booking practice courts.
For decades the consensus was that the very best coaches were court-savvy, former middle-ranked players who never achieved true glory in competition. Men such as Darren Cahill (career-high ranking of no.22 in the world), Larry Stefanki (world no.35), Paul Annacone (world no.12) and Brad Gilbert (world no.4).
Former world no.1 Jim Courier described it well when he talked of his (and Roger Federer's) former coach Jose Higueras (career-high ranking of no.6).
“He wasn’t a guy who had a lot of overt weapons as a player but, boy, he could understand the dimensions of the tennis court and translate it. It’s as much about your bedside manner as a coach."
Then came the 2010s, and the era of the celebrity tennis coach.
So Djokovic hired Boris Becker. Federer hired Stefan Edberg. Wawrinka hired Magnus Norman. Sharapova hired Jimmy Connors (albeit for about three minutes). Cilic hired Goran Ivanisevic. Gasquet hired Sergi Bruguera. Nishikori hired Michael Chang. Murray hired Ivan Lendl.
Then this year the coaching ante got well and truly upped as Murray hired not just a multiple Grand Slam champion, but a woman, to boot.
Now, Polish player Agnieszka Radwanska has upped the ante even further. By appointing Martina Navratilova as her coach, she has employed not only a woman, but one of the greatest Grand Slammers of all time.
"I did not sleep very well last night, thinking about getting back into match mode and the competitions," said the new coach after the responsibility of her role became apparent.
That's probably an exaggeration since a woman of her vast experience will have so much to offer a player such as Radwanska - a wealth of counsel well beyond bananas, practice courts and feeding balls. The Czech-turned-American won 18 Grand Slam singles titles, for Pete's sake.
For Radwanska, this isn't coaching so much as brand-building.
So where is this arms race heading? Who are we likely to see gracing the players' boxes in seasons to come?
Her are some hypothetical match-ups that may or may not bear fruit. (Take them all with the commensurate pinch of salt.)
The French connection: Jo-Wilfried Tsonga teams up with Yannick Noah to create a Gallic force majeure.
The two Johns: John Isner and John McEnroe. Isner already has the power. What he needs now is a bit of fire in this belly. Enter Johnny Mac.
Canada can: Canadian no.1 Milos Raonic hires Canadian-turned-Brit Greg Rusedski whose coaching work at Britain's Lawn Tennis Association has already borne a bit of fruit.
When in Romania: Simona Halep goes to her crafty compatriot Ilie Nastase and learns the fine art of gamesmanship.
Deutschland uber alles: Angelique Kerber and Steffi Graf. The German legend makes a rare step out of retirement in an attempt to lift her compatriot to the next level.
Brit hitters: Heather Watson and Virginia Wade. The British no.1 needs a kick up the backside if she is ever to make her mark on the sport, and Wade, both intelligent and incisive, has the boots to deliver said kick.
Will Navratilova have a substantial influence over Radwanska at the upcoming Australian Open? Unibet have her as 9th favourite to win overall at 26.00. Serena Williams is favourite at 2.62, followed by Petra Kvitova and Maria Sharapova at 6.00, and Simona Halep at 7.50.