For much of the last two decades, men's tennis has been obsessed with one question: How do you stop Roger Federer?
And after 20 grand slam titles and a record 302 weeks at world number one, it is clear few have identified the correct formula.
Even as Federer prepares to turn 37 the conundrum is becoming increasingly problematic, with the Swiss master on the brink of returning to the summit of the sport.
A semi-final finish or better at this week's Rotterdam Open will confirm Federer as the form player of the last 12 months and end a near six-year absence from the top of the ATP standings.
The likes of Philipp Kohlschreiber, Stan Wawrinka, Alexander Zverev and Grigor Dimitrov are likely to come up against Federer this week, each facing the same question hundreds have failed to answer down the years.
Former Wimbledon doubles champion Michael Stich has previously claimed he could beat Federer on his favourite surface of grass, where the great man has a record eight singles titles at SW19.
And the German right-hander believes only a relentless attacking approach is good enough to topple Federer, with Sergiy Stakhovsky's five-set win at Wimbledon in 2013 the blueprint for any player daring to go toe-to-toe with the veteran.
"We've seen odd matches like Stakhovsky beating Federer one year, where the guy's played serve-volley all the time and kept putting pressure and pressure all the time," Stich, who retired a year before Federer turned professional in 1998, told the Sports Illustrated Beyond the Baseline podcast.
"I have not seen any of the top guys these days hitting passing shots over five sets. I think they won't.
"I have a philosophy. If I lose the first set let's say 6-2, and I see that my tactics don't work, well I try something different. Because it doesn't matter if I lose 6-2 or 7-6 – as long as I lose, it's a bad result.
"So I might as well try something and give myself the chance to come out as a winner. When players came off court losing against Roger and saying 'it was a pleasure to lose against Roger Federer', that's where I feel like, wow, please.
"That's not the competition you want to have on the tennis court. No matter how good the guy is, just take some risks. Try to get away from your comfort zone and try something out. If you lose, well, make it better the next time."
The next player who will be hoping to heed that advice is Belgium's Ruben Bemelmans, whose reward for coming through the 16-man qualifying event in Rotterdam is a clash with Federer in round one.
Ranked outside the top 100 and without an appearance at the second week of a major, it remains to be seen whether the 30-year-old can trouble Federer sufficiently to end his hopes of returning to world number one prematurely.
Nevertheless, he will surely be hoping to channel the success of another left-hander – Rafael Nadal – when he steps out at the ABN AMRO World Tennis Tournament this week, with no player having racked up more than the 23 wins the Spaniard has accumulated against Federer during their respective careers.