'capable opponent' is a correct and usable phrase in written English.
You could use it to describe someone who is skilled at something and presents a challenge when in competition. Example: "My tennis game has improved a lot since I started playing against a capable opponent."
In the penultimate round, Carlsen played white against Sipke Ernst, a capable opponent more than ten years his senior.
If a more capable opponent had been at Madison Square Garden, such absent-mindedness might have cost the Knicks.
Illinois voters deserve to see a capable opponent force him to answer tough questions and defend his positions.
While it is hard to count him out, he has also struggled in the only two games in which Texas played a capable opponent this season.
But Donald Trump is, obviously, not a capable opponent.
A capable opponent would quickly turn any of those stories into a crisis for Clinton, using it to build out the caricature of her as a deceptive shill who's careless with state secrets and totally under the thumb of toxic moneyed interests.
Rutgers will have plenty of opportunities to prove that it can compete with capable opponents.
At the time Notre Dame played those four, all but Purdue were ranked by the subjective polls, meaning that they were judged capable opponents then and have proven so since.
Olson boasted proudly of Boies' cross-examination skills, saying, "he so unmercifully dismantled the opposing witnesses that the first day of trial, our very worthy and capable opponents decided that most of their witnesses couldn't make it to California".
They argue that on-alert land-based missiles are necessary because, without them, there would be fewer targets for a nuclear-capable opponent.
Anderson said online voting is fine in "low-stakes elections" – he cites its use at the Institute of Engineering and Technology, and at his own university council – but said it's "less fine where there are capable motivated opponents".
