Rotterdam - Roger Federer moved to within one victory of becoming the oldest man to take the world No 1 ranking on Thursday with a hard-fought 7-6 (10/8), 7-5 victory over Germany's Philipp Kohlschreiber at the Rotterdam Open.
The 36-year-old will now face either Robin Haase or Tallon Griekspoor in the quarter-finals on Friday.
Should he win that quarter-final, Federer would become the oldest No 1 male player, surpassing Andre Agassi, who holds the record at 33 years and 133 days.
Thursday's win was Federer's 13th in as many meetings against Kohlschreiber.
The first set was a 54-minute struggle with the top seed having to work hard against the veteran German who was broken for the only time in the match when he double-faulted to hand Federer a 6-5 lead in the second set.
Federer, the 20-time Grand Slam title winner, then eased to victory in the 12th game with a volley winner on second match point.
Second seed Grigor Dimitrov outlasted Serbia's Filip Krajinovic 7-6 (7/4), 7-5 to also reach the quarter-finals.
Dimitrov has now reached a second straight quarter-final in the Dutch city, advancing with 13 aces and saving all four break points he faced.
The Bulgarian will face Andrey Rublev on Friday after the 20-year-old Russian struggled for nearly two hours before finally seeing off Damir Dzumhur 6-4, 7-6 (7/4).
Rublev, the world number 34, forced his Bosnian opponent to save 10 of 15 break points while he saved nine of 13.
Russian Daniil Medvedev joined his compatriot in the last eight by beating Pierre-Hugues Herbert of France 3-6, 7-6 (7/2), 6-4.
The two-and-a-quarter-hour contest featured 13 aces from the loser and just four from the winner.
Results at the ATP Rotterdam World Tennis tournament on Thursday (x denotes seeding):
Roger Federer (SUI x1) bt Philipp Kohlschreiber (GER) 7-6 (10/8), 7-5
Robin Haase (NED) bt Tallon Griekspoor (NED) 6-4, 6-0
Daniil Medvedev (RUS) bt Pierre-Hugues Herbert (FRA) 3-6, 7-6 (7/2), 6-4
Andrey Rublev (RUS) bt Damir Dzumhur (BIH) 6-4, 7-6 (7/4)
Grigor Dimitrov (BUL x2) bt Filip Krajinovic (SRB) 7-6 (7/4), 7-5