Roger Federer's father Robert Federer has said his son's reign at the top of ATP men's singles rankings could be shortlived as he expects arch-rival and second-placed Rafael Nadal to return to the spot in a "few weeks".

The 2018 Australian Open winnner won the Rotterdam Open after defeating Grigor Dimitrov in the final. The Swiss tennis star eclipsed Nadal to the top spot when he made it to the semi-final of the ATP 500 event.

Federer became the oldest world number one in tennis as the 36-year-old surpassed Andre Agassi, who held the record for holding the top spot at 33.

With the win over Dimitrov in Rotterdam, Federer took his tally to 10,105 ranking points. He thereby extended the lead over second-placed Nadal to 345 points.

Nadal has not played since injuring his hip in the Australian Open quarter-final against eventual runner-up Marin Cilic in January. However, the Spaniard will be back in action in the ATP 500 tournament in Acapulco, starting Monday (February 26).

"How long he can defend the top spot now is in the stars. Maybe only a few weeks - if Nadal plays well in Acapulco - it can be over very quickly," Robert told the Swiss newspaper Blick.

File photo: Rafael Nadal of Spain plays a backhand in his Australian Open quarter-final match against Marin Cilic of Croatia on January 23, 2018.Michael Dodge/Getty

Can Nadal overtake Federer in Acapulco?

Federer is assured of staying at the top spot for at least for a couple more weeks as Nadal will not able to dethrone the former even he wins the title in Acapulco. The 16-time Grand Slam champion will be dropping 300 points, following his runner-up finish at the ATP 500 tournament last year.

On the other hand, the Swiss great will drop only 45 points ahead of the Dubai Duty Free Tennis Championships, starting February 26 as he had bowed out in the second round in 2017. There were doubts over his participation in the ATP 500 tournament but he ignited hopes last week when he reportedly said he is "still open" to playing in the United Arab Emirates (UAE).

The two might be involved in a direct battle for the number one spot at the season's first ATP World Tour Masters 1000, starting March 8 in Indian Wells. Notably, Federer defeated Nadal as early as in the fourth round of the same tournament in 2017.

Unprecedented response

Meanwhile, Robert opined the world number one tag was important to Federer, but expressed surprise at the unprecedented reception his son received for just winning the ATP 500 tournament in Rotterdam.

Federer himself said getting back to the top spot of ATP rankings after a five-year gap "meant the most" to him.

"Of course it is great. World No 1 has always been important to Roger... It's just a 500 ATP tournament - not a grand slam - and yet the attention was huge," Robert said.

He added: "I received three times more messages, mails and calls from Europe, USA, Africa and Australia than after his wins at Wimbledon or at the Australian Open!"