Roger Federer's return to world number one is arguably the biggest "milestone" of his professional career, according to his father Robert Federer.

Victory at the Rotterdam Open helped the 36-year-old return to the summit of the ATP rankings for the first time for five-and-a-half years and become the oldest man to do so in tennis history.

The Swiss legend has been lauded for the achievement and despite previously spending in excess of 300 weeks as world number one and winning 20 grand slam titles Federer senior feels this could be the pinnacle.

"This number one is probably the milestone in his career," he told local publication Blick. "Everything was fine again, before the wheel will inevitably stop at some point.

"Of course that's great. World number one has always been important to Roger. Of course I've already congratulated him by phone, we're always in contact with Roger."

Federer decided to accept the offer of a wild card from tournament organisers just a week before the event, one which he has not played since 2013, and now looks set to skip next week's Dubai Duty Free Tennis Championships which he appeared at last year.

Wins over Ruben Bemelmans, Philipp Kohlschreiber, Robin Haase, Andreas Seppi and Grigor Dimitrov maintained Federer's unbeaten start to 2018 and helped add his second title of the season, having won the Australian Open in January.

But despite the inferior nature of the event, Federer's father was inundated with well-wishers upon his son adding another glittering achievement to his career CV.

"That's just a 500 ATP tournament, not a grand slam," he added. "And yet the attention was huge - 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."