As an example, consider this sentence:
When Roger Federer plays tennis, he just "makes it look so easy".
A single word or maybe a smaller phrase that I could replace it with?
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In addition to the other answers which have discussed your specific example, if you want to generalize a task is easy for someone expert at it, the popular phrases are
That was a "piece of cake" for him
That was "easy as pie" for him
and a single word,
That was a "cakewalk" for him
Examples from the link above -
"winning the tournament was a cakewalk for him"; "invading Iraq won't be a cakewalk"
I don't think there is such a one word version of that idea in English.
To say Roger Federer plays tennis "effortlessly" is decidedly NOT the same thing as him making it look easy. In fact, the magic of some gifted athletes--this is often said of figure ice skaters--is that they make something that is, even for them, obviously difficult and full of effort look easy.
@Chris B. Behrens correctly identifies the intended meaning in OP's context, but the question title is at the very least ambiguous.
We can assume that when Roger Federer plays tennis, his primary concern is to win the match. He's not really interested in showing others how easy it is to become a world-class player. OP's sentence is a simple description of how he plays (or appears to play, since I'm sure in reality it's not at all easy, even for him).
On the other hand, if a skilled teacher makes it look easy he is doing something totally different, and he may in fact need to put considerable effort into successfully demystifying his subject.
A few other options are:
to perform in a facile, fluent, or adroit manner,
or to perform with aplomb, facility, fluency, or composure.
In some contexts you could even use nonchalant or insouciant.
Even though it may not directly be a good answer to the OP's question, I thought it would be good to mention the Dictionary.com word of the day: ballon, which was the impetus to pose myself the same question the OP did. This refers to the quality a skilled ballerina possesses to be able to appear to float.