In a continuing quest for lexical items that cross over sports, I have resorted to reading summaries of tennis matches, which has me slapping my forehead for not having noticed the use of ace: tennis, cards, dice, golf, and beyond. But this one's trajectory is not at all obvious, since it's associated with games whose origins are far more ancient than those that give us home runs and touchdowns.
OED puts the earliest use of ace as the side of a die with one point value, and tracing the word from Old French as, from Latin as, meaning unity or unit. The earliest citation is from c. 1300: Harrowing of Hell 21 Stille be thou, Sathanas! The ys fallen ambes aas. (ambes aas = two aces, a.k.a. snake eyes). The earliest citation in a card-playing context is from 1533.
The tennis use of ace means "unreturnable serve", and its notion of one-ness seemingly relates to the earning of a point with one stroke. OED has 19th-century citations to this effect, with earlier uses referring more generally to any point in any volleying game. Also notably, Oxford has a 1920 citation of ace used for a hole-in-one in golf.
Outside sports and games, it can mean "one who is good at what they do". OED's earliest reference is to skilled fighter pilots in WW1, noted cryptically "after F. as". Its extension outside military usage is "chiefly US". Other chiefly US usages of ace are a slang term for dollar, and a verbal usage, meaning to earn an excellent score on a test or exam.
Here's what's not immediately clear: (1) is the "unreturnable serve" usage extended from the dice/cards usage, and if so, in French or English? (2) how did the "expert" usage come about? Reading the OED entry, I got the faintest hint that the "unreturnable serve" is an English innovation, but the "skilled one" is a second borrowing from French. Intrigued, I took a walk to the library, looking for French resources. (If you read French you'll see my interpretations below are a bit loose).
I consulted numerous dictionnaires etymologiques, the least helpful of which said the following:
As, du L. as (meme sens). (As, from Latin as, same sense).
The others seemed to agree that as took on a dice role first, jumping to cards, but differ in their stories of how the "expert" usage came along. Caillon (1962) mentions it, but makes no explicit connection:
AS [ass] (du l. as unité de monnaie, de poids), carte a jouer qui n'est marquée qu'un point. Fig. pop.: le premier dans son genre. (From Latin as, a unit of money or weight; playing card worth one point. Slang: the best in one's field.)
Bloch & von Wartburg (1932/1968) assert that the "expert" usage extends the winning position of the ace in a deck of playing cards:
As. Lat. as «unité de monnaie, de mésure»; a dû être employé de bonne heure pour le jeu de dés, d'où postérieurement pour le jeu des cartes. As «homme de valeur» partic. dans les sports, d'après la valeur de l'as dans les jeux de cartes, est du XXe s. (Lat, unit of money or measure, soon to be used in dice, and later for playing cards. As "one who is strong", particularly in sports, after the value of the ace in cards, is of the 20th C.)
However, Dauzat (1938) traces the "expert" usage through a military application (as hinted at by OED):
as (XIIe s., var asse, XVIe s.). empr. au. lat. as, unité de monnaie, de poids; en fr. terme de jeu de dés, puis de cartes; au fig. «cavalier du premier péloton», argot milit. (début du XXe s.), puis «soldat de valeur» (sens développé par la guerre) et, par ext., «homme de valeur»; le sens pop. as, plur., «argent», vient d'as, carte maîtresse. (from Latin as, unit of money or weight; in French, term of dice, then of cards; figuratively to "rider at head of column", military usage, then "skilled soldier" (developed during the war), and by extension "skilled person". Slang as, plural, "money", comes from ace as a winning card).
So, several neat outcomes:
(1) As in French and ace in English reacquired a meaning of money that as once had in Latin.
(2) The "unreturnable serve" is an English extension of ace. Except for very recent borrowing back to French from English, French as in a tennis context refers to a good player rather than a good serve. (I found a French tennis lexicon online with the search term marquer un ace, in which ace is listed as follows: balle de service que le relanceur peut toucher, mais ne peut relancer dans les limites du court. Réussir un ace permet de gagner un point avec une balle de service (service ball that the receiver can touch, but cannot return in bounds. Getting an ace scores a point with the serve). But ace is not in any print French dictionary I consulted, and no entry for as mentions this usage.
(3) As, as "head of column", is French for point man.