I've had a look online and can't find any evidence that the term "Ace" refers to a position in volleyball; the only definition I could find was that of a service ace.
Is this term solely used for the sake of the anime/manga or is there any evidence that points to its use in real life?
It's not a "position" in the same sense that "libero" and "middle blocker" are positions. That is to say, you probably aren't going to find any references to an "ace" as a position in, say, the Japan Volleyball Association's rulebook.
When Haikyuu!! refers to Azumane as Karasuno's "ace", that's in the usual colloquial sense - he's the best (offensive) player on the team. In English, this sense of "ace" seems to be fairly restricted to baseball (where it refers to the best pitcher on the team), but the Japanese equivalent (エース ēsu, borrowed from English "ace") is used fairly often in real life for team-leading players in other team sports as well, including (at minimum) soccer and volleyball.
As it turns out, this turn of phrase has made it way into other parts of Japanese volleyball lingo. A wing spiker (Eng. "outside hitter" or "opposite hitter") who focuses primarily on offense may also be referred to as a "super ace". But this is not the sense in which Haikyuu!! tends to use the term "ace".
The player who scores the most, being the ace, is a opposite hitter or attacker...
http://www.volleyballadvisors.com/volleyball-positions.html
This pretty much explains the positions played in volleyball and it’s easy to understand what they actually mean when they call him “Ace” it’s almost always in any sport that uses the title; the player who scores the most.
From the position stand points, the ACE seems to be the right-side hitter (in volleyball terminology) that is directly opposite of the setter. They are the strongest blockers because they have to block the ”wing spiker” or left-side hitter which is typically their best hitter. They are typically left handed, tall, and able to run shoots, 9, and 7s (behind the back sets usually). If they are really good hitters, it’s a HUGE advantage to the team because the opponents will have a weaker block on that side, considering it’s only occupied when the setter is in the back row. But they also might just mean, most reliable hitter.
Ace may depend on country but from what I gathered it’s actually the outside hitter that is the ace. Meaning the position that is on the left hand side. I used to play volleyball and I know in America we don’t use that term but we do rely heavily on our outside hitter to get us out of a bind since they’re supposed to be the best well rounded player as well as the best hitter. Americans don’t have a term for it but it’s possible that Japanese use it to encourage within the team.
