World number five Alexander Zverev believes even Roger Federer has found his success since the beginning of last year to be a surprise.
Federer returned from an injury lay-off to win the 2017 Australian Open in what was his first Grand Slam win since 2012.
The Swiss ace would continue to defy the odds and his age to win another six titles, including a record-breaking eighth Wimbledon crown and eventually end the year as the world number two, having started it at 16th.
Having won the Australian Open this year as well after his five-set win over Marin Cilic last month, Federer now has the opportunity to become the world number one this week.
The 36-year-old accepted a wild card into the Rotterdam Open, where if he reaches the semi-final stage, he will become the oldest world number one in history, having last held the top ranked position back in November 2012.
In addition, he will break Rafael Nadal's record for the longest time between first becoming a world number one and the last time they held it.
Zverev, who is also taking part in the tournament, had nothing but praise for the 20-time Grand Slam champion who he could coincidentally meet in the semi-final stage.
"It's great," Zverev said in a video interview on the ATP World Tour website. "He might be the world No 1 after the week so it's obviously great to see him with such success still.
"I don't think even he could have expected that 13, 14 months ago when he was outside the top 10, winning three majors in the last five.
"It's great so I wish him the best of luck and hopefully he can get to the semi-finals and we'll see how everything else will go."
Federer will face Ruben Bemelmans in his opening match on Tuesday (13 February) and could face his compatriot in Stan Wawrinka in the quarter-final stage.