Roger Federer will become the oldest man to ascend to world number one if he defeats Robin Haase at the Rotterdam Open on Friday.
The Swiss maestro, who last held the top ranking in November 2012, will return to the summit if he reaches the semi-finals at the ATP 500 event.
After seeing off Ruben Bemelmans in the first round, two-time champion Federer came through 7-6 (10-8) 7-5 against Philipp Kohlschreiber in a hard-fought battle on Thursday.
That secured his place in the last eight, before Haase overcame fellow Dutchman Tallon Griekspoor 6-4 6-0, fighting back from a break down in the opener and rescuing three break points in the first game of the second before cruising through.
Second seed Grigor Dimitrov also enjoyed safe passage although, like Federer, he had to fight to oust Filip Krajinovic 7-6 (7-4) 7-5.
Dimitrov will meet Andrey Rublev in the quarters after the Russian got the better of Damir Dzumhur 6-4 7-6 (7-4), while Daniil Medvedev came from a set down to defeat Pierre-Hugues Herbert 3-6 7-6 (7-2) 6-4 to set up a clash with Andreas Seppi.
The quarter-finals are scheduled to take place on Friday, when all eyes will be trained on Federer as the 20-time grand slam champion bids to create yet another slice of history.