Roger Federer’s coach Ivan Ljubicic admitted coaching the Swiss maestro is "very complicated" as it is difficult to improve a player widely considered the greatest ever to play the game.

The 36-year-old has won 20 men’s singles Grand Slam titles, holds the record for the most number of weeks as the world number one and is second on the list for number of singles titles won – his 97 ATP singles titles is only behind Jimmy Connors, who has 109.

Ljubicic joined Federer’s coaching team in 2016 following the departure of Stefan Edberg and was part of one of his most dominant periods in the game. Federer won 17 of his 20 men’s singles Grand Slam titles between 2003 and 2012 after which he went through a period of drought.

Roger Federer Roger Federer has won 20 men's singles Grand Slam titles. In this picture, Federer (L) of Switzerland and coach Ivan Ljubicic pose with the Norman Brookes Challenge Cup in the players locker room after winning the Men's Singles Final against Marin Cilic of Croatia on day 14 of the 2018 Australian Open at Melbourne Park, Australia, Jan. 28, 2018. Photo: Clive Brunskill/Getty Images

The Swiss ace failed to win a single major title between 2012 and 2016, despite making three finals. He then spent the last six months of the 2016 season on the sidelines due to a knee injury, when questions about his future in the game crept up.

A number of former players predicted the end for Federer suggesting he was unlikely to return to the top of the game. However, he proved his doubters wrong with an explosive return in 2017 when he picked up seven titles including two Grand Slams and made his way back to world number two.

The 20-time Grand Slam champion carried his momentum into 2018 and defended his title at the Australian Open and then created history when he became the oldest world number one in ATP history, following his win at the Rotterdam Open.  Ljubicic has been part of this period, when Federer has defied logic and dominated the sport, which could be part of the reason the Croatian suggested it is complicated to coach the Swiss star.

“My work with Roger is complicated , it's very complicated to manage to improve such a player,” Ljubicic said about his partnership with Federer, as quoted by Tennis World USA. “But the easiness, transferring a simple idea to the player covers an important part.”

Ljubicic also spoke about the younger generation coming through in the game, more specifically about his compatriot Borna Coric. The 21-year-old has already recorded a pair of wins over Andy Murray and Rafael Nadal in his short career, but Federer’s coach is keen to see if the younger generation can beat the top players on a consistent basis rather than just in one of events.

“They are close. Borna (Coric) already beat (Andy) Murray and (Rafael) Nadal and made Roger struggle a lot in Indian Wells by playing so well,” Ljubicic said. “But it is one thing to play a match well, to win or to get closer to beating them, but it is another thing to beat them several times, on the most important stages like Grand Slams.”