das Ass
ace
The playing card with the highest or lowest value in many card games, usually marked with the symbol A.
Beim Skat ist das Kreuz-Ass der höchste Trumpf.
In Skat, the ace of clubs is the highest trump card.

ace
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ace
The playing card with the highest or lowest value in many card games, usually marked with the symbol A.
Beim Skat ist das Kreuz-Ass der höchste Trumpf.
In Skat, the ace of clubs is the highest trump card.

Careful: 'das Ass' (ace) has nothing to do with the English vulgar word 'ass.' In German, it is either neutral (a playing card) or positive (an expert, a point in sports). Confusing the two is a very common mistake.
Mit vier Assen auf der Hand hat er das Spiel gewonnen.
With four aces in his hand, he won the game.
When 'Ass' describes a person who is very good at something, use the preposition 'in' followed by the dative to name the field.
Meine Schwester ist ein Ass in Mathematik.
My sister is an ace in mathematics.
Calling a person an 'Ass' (ace) is colloquial. In formal or written contexts, you would more likely use words such as 'Experte' (expert) or 'Könner' (skilled person).
Frag mal Paul, der ist ein echtes Ass bei Autoreparaturen.
Ask Paul—he's a real ace at car repairs.
In a sports context, especially in tennis or volleyball, 'Ass' is combined with the verbs 'schlagen' (to hit) or 'servieren' (to serve).
Der Tennisspieler hat im letzten Satz drei Asse geschlagen.
The tennis player hit three aces in the last set.
service
'Service' and 'Ass' can both have to do with the serve in sports, especially tennis or volleyball. 'Service' means the serve itself, that is, the shot that starts a rally. 'Ass' means only a special kind of serve: the opponent cannot reach the ball or cannot return it properly, and the point goes directly to the person serving. The words can be confused because every ace is a serve, but not every serve is an ace. If you just mean the serve, use 'Service'; if the serve wins a point directly, use 'Ass'.

Mit einem Ass gewann die Spielerin den letzten Punkt des Spiels.
The player won the last point of the match with an ace.
Beim Tennis übt Paul vor dem Turnier seinen Service.
In tennis, Paul practices his serve before the tournament.