Novak Djokovic of Serbia returns the ball to Feliciano Lopez of Spain during their match at the ATP Dubai Duty Free Tennis Championships February 25, 2016. Reuters/Ahmed Jadallah

Serena Williams' coach Patrick Moratoglou thinks Novak Djokovic will complete a calendar Grand Slam first before Roger Federer wins another major title or Rafael Nadal gets back to his elite form.

In an interview with Tennis Best, Mortagolou was asked who among Djokovic, Federer and Rafael Nadal would achieve their ultimate goals this season. The Serb 'Djoker' is gunning for a calendar sweep this season, while the Swiss Maestro Federer is looking to extend his all-time record Grand Slam wins. As for Nadal, he is hoping to return to top form after a season full of struggles last year.

The 45-year-old French coach of WTA’s world No. 1 Serena Williams, believes world No. 1 Djokovic will be the first to accomplish the ever-elusive rare feat of a complete calender Grand Slam win before Federer or Nadal realise their own dreams.

“The Grand Slam is an exploit amazingly difficult to realise but if Djokovic keeps playing this way... the only one who can stop him, is himself, [or] if he has a bad day and his opponent plays the match of his life,” Mortagolou said, as translated by Tennis World USA.

Djokovic was close to completing a calendar Grand Slam last season, but fell short after losing in the final round of the French Open against Stan Wawrinka. The Serb won four of the last five Grand Slam events, including this year’s Australian Open, but has suffered a slight setback over an eye problem at the Dubai Tennis Championships.

"My match record is the least of my thoughts at this moment," Djokovic said, reports ESPN. "I just hope that this problem will fade away in the next few days."

On the other hand, Federer has not won a Grand Slam since 2012 at Wimbledon. The 34-year-old Swiss was close to winning last year’s edition of the tournament and the US Open, but was defeated by Djokovic at both events.

Meanwhile, Nadal's road is the same story so far as he struggles to find his rhythm on the hard court. After promising performances in the last four tournaments of 2015, the “King of Clay” failed to carry his momentum into this season, suffering upsets at the Australian Open, Argentina Open and Rio Open.

Djokovic and Nadal are expected to meet at Indian Wells, but Federer, who is recovering from a knee surgery that repaired a left torn meniscus will miss the tournament and instead return at the Monte Carlo-Rolex Masters in France.