It will be hard for anyone to surpass Roger Federer and Rafael Nadal in 2018, according to the former's ex-coach.
Federer won the Rotterdam Open after beating Cilic in the Australian Open, picking up his 97th title in his illustrious career.
Federer decided to play less, change his racquet, shorten his matches and play even more forward so as to not exert himself beyond his capacity.
Federer has made history by becoming the oldest world number one, eclipsing the record set by Andre Agassi way back in 2003.
Federer leapfrogged Nadal in the world rankings after reaching the semi-finals of the Rotterdam Open.
Andre Agassi has revealed which players he feels are capable of challenging Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray.
Federer became the oldest world number one in tennis following his quarter-final victory over Robin Haase at the Rotterdam Open.
Federer skipped the entire clay court swing of the season last year following a blistering start.
Roger Federer and Grigor Dimitrov could face each other for the seventh time if they win their respective semi-final clashes at the 2018 Rotterdam Open.
Roger Federer became the oldest world number one after reaching the semi-finals of the Rotterdam Open in remarkable fashion.
Federer has beaten all odds at the age of 36 despite people writing off his chances to make the top of the ATP rankings.
Roger Federer has not lost in 2018 having defended his crown at the Australian Open and is now in the semi-finals at the Rotterdam Open.
The Swiss ace came from being a set down to beat Robin Haase in the quarter-finals of the Rotterdam Open.
Marin Cilic lost to Roger Federer at the 2017 Wimbledon final and 2018 Australian Open final. Tracy Austin believes Cilic can finish the year as the new No.1 if he is consistent.
Roger Federer became the oldest world number one in tennis after his quarter-final win over Robin Haase at the Rotterdam Open.
Roger Federer defeated Robin Haase in the quarter-finals of the Rotterdam Open to become the oldest world number one.
The 35-year-old Swiss legend eclipses Andre Agassi as the oldest No.1 in tennis history after overcoming Robin Haase.
Federer is looking forward to becoming the oldest World No 1 eclipsing Andre Agassi's record at Rotterdam.
Roger Federer will become the world number one if he beats Robin Haase in the quarter-finals of the Rotterdam Open on Friday (16 February).
Roger Federer will beat Rafael Nadal to the number one spot should he make it to the semi-finals of the Rotterdam Open.
Roger Federer, Rafael Nadal and Novak Djokovic have dominated the men's game for more than a decade.
Federer played Ruben Bemelmans on 14 February, dismantling him in straight sets, 6-1 6-2, in a game which lasted just 46 minutes.