Roger Federer is one win from becoming the oldest world number one in ATP Tour history after beating Philipp Kohlschreiber 7-6(8), 7-5 in the last 16 of the Rotterdam Open on Thursday.
The 36-year-old needs to reach the semi-finals to overtake Rafael Nadal at the top of the rankings and reclaim the world number one ranking for the first time since November 2012.
Evergreen Federer, the 20-time Grand Slam champion, faces local favourite Robin Haase in the last eight on Friday.
Kohlschreiber, 34, had not beaten Federer in 12 attempts but on his own service game proved a tough opponent for the Swiss, pushing him all the way to a first set tiebreak.
The German even had two set points but could not convert them as Federer rallied and placed a forehand perfectly beyond Kohlschreiber to take the lead in the match after 54 minutes.
The second set followed a similar pattern but with Kohlschreiber, ranked 36 in the world, more attuned to Federer's serve and improving his return rate.
He still had to save four break points without getting close himself to threatening the Australian Open champion's serve.
Kohlschreiber's resistance was finally breached in the 11th game as Federer overpowered his opponent and went on to comfortably serve out the set.
Federer would surpass American Andre Agassi as the oldest world number one if he wins his next match in Rotterdam and he would also set another record of the longest gap (five years and 106 days) between periods as number one.
He already holds the record for most weeks spent at number one, having topped the rankings for a combined 302 weeks.
Wozniacki and Halep ease into Doha quarters, Pliskova crashes out
Caroline Wozniacki and Simona Halep cruised into the quarter-finals at the Qatar Open on Thursday, while defending champion Karolina Pliskova was knocked out by qualifier Catherine Bellis.
World number one Wozniacki, who won the Australian Open last month, survived a topsy-turvy opening set to beat Romania's Monica Niculescu 7-5, 6-1.
Wozniacki raced to a 4-1 lead but then struggled against Niculescu's crafty slices to lose four consecutive games.
The Dane regained her composure and went on to drop just one more game to book a quarter-final spot after an hour and 30 minutes of play on centre court.
Earlier in the day, Halep shrugged off a slow start to beat Latvian Anastasija Sevastova 6-4, 6-3, keeping her hopes of regaining the number one ranking alive this week.
The 26-year-old Romanian rallied back from an early break and remained aggressive throughout, landing 29 winners to 21 unforced errors.
She converted five of eight break point opportunities to reach the last-eight in Doha for the first time since 2014.
Should Halep finish the tournament with one win more than Wozniacki, she will reclaim the top spot she lost after the Australian Open on Monday's ranking.
Halep will next face American CiCi Bellis, who claimed the biggest win of her career after overcoming world number five Karolina Pliskova 7-6(4), 6-3.
Bellis looked fairly comfortable against her Czech opponent's powerful serve, breaking her five times on her way to victory in an hour and 37 minutes.
"This was huge for me," 18-year-old Bellis, ranked 48 in the world, said.
"I think I played a really good match and just tried to track down as many balls as I could. Obviously, I've watched her play for many years and she's one of my idols for sure. I'm just really happy to was able to get through it."
