Roger Federer became the oldest number one in tennis history with a quarter-final win over Robin Haase at Rotterdam in February 2018.
The 36-year-old went on to beat Andreas Seppi in the semi-finals before earning a straight-sets win over Grigor Dimitrov in the decider to claim his 97th career title.
However, the Swiss veteran only maintained top spot for six weeks after losing to Thanasi Kokkinakis in the second round of the Miami Open, conceding top spot to Rafael Nadal.
Later, Federer grabbed it back for a week, before Nadal snatched it back for four weeks - during which he won his 11th Roland-Garros crown - before the Swiss ace had it for another week.
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In the July update, Nadal regained the top ranking from Federer, meaning the top spot had been traded by the pair an extraordinary six times in 2018. That became seven when Novak Djokovic ascended the mount once again.
Federer skipped the clay court season in 2018, like he did last in 2017, with hoped an all-out attack on the grass court season would culminate in a ninth Wimbledon crown. It didn't happen, with Djokovic beginning a brilliant run as he also won in New York and Melbourne.
Here’s a quick recap of the oldest men’s world number ones.
Who is the oldest world number one?
Roger Federer was 36 years and 195 days old when he defeated Robin Haase in the quarter-final at Rotterdam to become the oldest ever world number one.
Most recently, Federer was 36 years and 320 days old when he last held the top ranking in June.
Rafael Nadal holds the record as the oldest player to finish the year as the top-ranked player - the Spaniard was 31 at the end of the 2017 season.
Who is the second oldest world number one?
Andre Agassi is the second-oldest world number one. The American was 33 in 2003 when he reached top spot again, taking the number one mantle from Lleyton Hewitt.
Agassi remained in that spot for 12 weeks before Spaniard Juan Carlos Ferrero took it off him.
Other oldest tennis world number ones
Rafael Nadal is third on the all-time list of oldest ATP number ones at 32 years of age.
Jimmy Connors is the next oldest world number one. The temperamental American was 30 when he reached the pinnacle for the last time in 1983.
Ivan Lendl rounds out the top five oldest number one men. He was 30 in 1990 when he became number one for the eighth time.
Andy Murray and John Newcombe were also 30-year-old world number ones.
Who are the oldest WTA world number ones?
Serena Williams is the oldest woman to ever reach number one spot. She was 35 when she traded the number one position back and forth with Angelique Kerber in 2017.
The 23-time Grand Slam champion is the second oldest world number one ever - behind only Federer - and could make another run at the top spot if her return from her pregnancy gains traction.
Chris Evert is the next oldest women's number one, at 30 years and 11 months old. Martina Navratilova was 30 years and nine months when she was world number one, while Lindsay Davenport was 29 the last time she was world number one. Her eighth and final reign came eight years after her first stint at the top of the game.
What is the longest gap between stints as world number one?
Federer holds this record as well, having last reached top spot on November 4 in 2012 prior to his reascension in February last year.
Agassi is in second spot, with a three year and 142-day gap between spots at the top of the game.
Connors has the third largest gap at three years and 65 days, while Nadal's three years and 45-day gap is the fourth longest.
Agassi makes another appearance at fifth, with a two year and 228-day hiatus.
What is the biggest gap between first and last appearance as No.1?
That's Federer as well - his first spell as world number one was a whopping 14 years ago.
Nadal is next, with an almost 10-year gap.
Connors at eight years and 339 days is next, with Agassi (eight years and 150 days) in fourth.
Sampras rounds out the top five, with a seven-year 221-day gap.
Who has spent the longest at number one?
Federer has spent the longest time in the number one spot, with 310 total weeks.
Sampras is second at 286 weeks, Lendl is third with 270 weeks and Connors is fourth at 268 weeks.
Incumbent No. 1 Djokovic is fifth and is the next longest standing current player with 241 weeks, with Nadal in sixth with 196 weeks, and McEnroe in seventh with 170 weeks.
Seasons with at least five changes to top ranking
1983 - 10 (Jimmy Connors, John McEnroe, Ivan Lendl)
1999 - 8 (Carlos Moya, Pete Sampras, Yevgeny Kafelnikov, Andre Agassi, Patrick Rafter)
2018 - 7 (Rafael Nadal, Roger Federer, Novak Djokovic)
1984 - 6 (Ivan Lendl, John McEnroe)
1992 - 5 (Jim Courier, Stefan Edberg)
1996 - 5 (Andre Agassi, Thomas Muster, Pete Sampras)
2001 - 5 (Marat Safin, Gustavo Kuerten, Lleyton Hewitt)
2003 - 5 (Andre Agassi, Lleyton Hewitt, Juan Carlos Ferrero, Andy Roddick)