Sitting in Limbo: Last year's young Wimbledon semifinalists no closer to breaking through

The up-and-comer is one of the great ciphers in sports. He's a vessel for projection - of envy, hope, and, most often, disappointment. He is a chunk of unmolded clay. He is potential made flesh.

It's exhilarating when that potential starts to take shape and maddening when it remains static. For Milos Raonic and Grigor Dimitrov, two wildly different tennis players lumped together by the parallel chronology of their professional careers, the thrills and frustrations have come in equal measure.

Raonic and Dimitrov have long headlined the group expected to succeed the incumbent tennis royals, to step into the power vacuum left (eventually, one presumes) by the Big Four, and carry tennis into its uncertain future. They've been the de facto Next Generation for so long, in fact, that it's hard to remember exactly when or why anyone felt the impetus to anoint them as such.

Each had a hook. Raonic had his rocket-armed serve, which he could send screaming up the T at 140 mph or spinning out high and wide and well out of reach. Dimitrov was a chameleonic throwback who oozed flash - a worthy enough heir to the bravura and aesthetic wonder of Roger Federer that people took to calling him "Baby Fed."

But the whens and hows of their presupposed takeover have remained out of focus. Federer and Rafael Nadal may have peaked some time ago, but they continue to contend. Novak Djokovic and Andy Murray are better than ever. And behind them, others lie in wait - gatekeepers like David Ferrer and Tomas Berdych, gate-crashers like Stan Wawrinka, Kei Nishikori, and Marin Cilic.

The gulf separating the best from the rest may have shrunk (Djokovic's personal island notwithstanding), but the ambitious expansionists bridging the divide are stepping over Raonic and Dimitrov in the process. They're still young, these two - each 24 - but it no longer makes sense to call them up-and-comers. They've come up. Here they are. They're just stuck in the mud; mired in various states of arrested development.

Their joint 2014 Wimbledon breakthroughs, in which both made it through to their first major semifinals, was supposed to herald their arrival. A year later, as they prepare to reprise their success and hang onto all those rankings points they earned at the All England Club, the lights have dimmed. Neither has been back to a major semi since, and, more worrisome, neither has given any indication he's continued to evolve.

Raonic has shown greater consistency, on the whole, but his ceiling also appears lower. The optics, whether instructive or not, aren't kind to him. He's lanky and flat-footed, his style unimaginative and uninspired, his one-dimensionality bordering on gimmickry.

He's shown, of course, he can be more than just a serve, an arm sleeve, and a slick hairdo. He's improved in subtle but meaningful ways over the years; his backhand is less of a liability and he's less reliant on his slice from that side, he's turned his overhead into a terror, he's moving up to net more frequently and more effectively.

And yet, he still doesn't have the footwork to hang in long rallies, or maintain good court position, or run around his backhand as often as he probably should. His serve and forehand are good enough that he almost always beats the guys he's supposed to. But he's shown no sign that the wall he keeps running into is going to come down any time soon.

Dimitrov is more enigmatic, because he seems to have all the tools. He's creative, he moves well, hits with variety, can flatten out his groundstrokes, finds angles that open up the court. But he's been plagued by inconsistency, by untimely errors, by mystifying mental lapses, and lags in energy.

There are more tangible issues. He isn't overpowering, lacks a dominant serve, and hasn't figured out how to weaponize his one-handed backhand. But those shortcomings should be surmountable for one with a game as variegated as Dimitrov's - just ask Nishikori. You watch and scratch your head at his inability to put it all together. He doesn't seem to know either.

"I felt I have been unlucky in the past weeks with a lot that has been going on," Dimitrov, who's lost four of his last six matches, said last week. "And it's something that, actually, I accept and I need to fight through."

He and Raonic will look to rediscover some of their Wimbledon magic. They're defending heaps of rankings points, but mostly, they're just trying to kick their seasons - and careers - out of neutral.

For now, they still seem to exist in the abstract; as potential made flesh. That potential may start to take shape, or it may remain forever amorphous. Futures, even the most promising, are fickle things.