Roger Federer said he "never imagined" he would be world number one again as he moved to within one victory of being the oldest man to achieve that feat.
The Swiss 20-time Grand Slam champion beat Germany's Philipp Kohlschreiber 7-6 (10-8) 7-5 to reach the quarter-finals of the Rotterdam Open.
If Federer, 36, beats Robin Haase in their last-eight tie on Friday, he will replace Rafael Nadal at the summit.
"I've struggled to try and get there. I had to win a lot of matches," he said.
Federer, who became world number one for the first time in February 2004, has not topped the rankings since October 2012 and slipped to a low of 17th in January 2017.