The first of those instances was in the final few years of the 2000s when a little-spoken-about Novak Djokovic was starting to challenge superstars Roger Federer and Rafael Nadal.
The Serb of course would go on to join the two at the pinnacle of the sport as their equal and ultimately surpassed them, but he was once very much in their shadow.
Back at the start of 2008, Federer and Nadal were seen as indisputably the 'Big Two' of men's tennis, sharing each of the last 11 Grand Slams between them as they dominated the sport.
Heading into the Australian Open that year, they were widely expected to clash in the final for the third time in four slams, but third seed Djokovic had other ideas, surprising everyone to beat top seed Federer in the semis.
It was a result that few saw coming and few fans wanted, denying them the chance to witness a dream final. Sound familiar?
As Federer was then, Djokovic is now the biggest name on the tour and - statistically at least - the GOAT. As Nadal was then, Carlos Alcaraz is the young challenger setting the world alight. And Daniil Medvedev is to them what the Serb himself was to 'Fedal'.
All of the talk heading into this year's US Open was about a potential rematch of the stunning Wimbledon final between Alcaraz and Djokovic, and the vast majority of people were confident they'd be the last two standing in Flushing Meadows.
Despite the fact that he had won that very tournament two years prior and headed into this edition with more hard-court titles than anyone in 2023, few gave Medvedev a serious chance, expecting him to lose in the semis to Alcaraz at best given how easily the Spaniard had beaten him at Wimbledon.
That remained the case even after the Russian produced impressive performances to knock out Alex De Minaur and Andrey Rublev but he finally stepped into the spotlight and captured people's attention with one of the best displays of the year in the final four.
He didn't just beat Alcaraz but fully deserved to, too, playing a near-perfect match in which he served excellently, hit some stunning winners and upped his game at the most important points to win in four sets.
The world number three couldn't produce such magic again in the final - he's only human - losing in straight sets to Djokovic, but he nonetheless showed in New York that the Serb and Alcaraz don't just have to worry about each other going forward.
His strong showing in the States was the latest of a number of steps forward the 27-year-old has taken in 2023. He's had his best season yet in terms of titles, winning five, and has for the first time in his career looked good on all three surfaces, winning the Rome Open on clay and making the Wimbledon semis on grass.
In the process, he's distinguished himself as the third-best player in the world ahead of the likes of Stefanos Tsitsipas, Holger Rune, Casper Ruud and Jannik Sinner, being far more consistent than all of them over the course of the year.
Djokovic had a strikingly similar season back in 2007, winning five titles - with four being on hard courts and one being on clay, too - to secure the world number three spot. And yet, being not quite as good and not as entertaining to watch, he remained firmly in the shadow of the two above him - and that's very much the case for Medvedev now.
Given he's a lot older now than the Serb was then and - if we're being honest - not as talented, he won't go on to usurp the two men he's chasing as Djokovic ultimately did. However, if he keeps playing the way he is, he will at least cause them serious problems going forward - most likely more than anyone else in the short term - which is an achievement in itself.
As was once the case with Djokovic, Medevev is not as loved and not as lauded as the sport's current Big Two. However, while they'll be the ones to write the history books that will cover this era in years to come, he'll at least get a few lines in.
