While I like Jeffrey Carney's answer (and upvoted it), I will write my own because I believe one thing about the question should be made clear. While the questioner's sentence may seem confusing or ambiguous to some, it is not grammatically incorrect. It would be clearly understood by native speakers under most circumstances.

However, as a matter of style, this sentence may not constructed in the clearest way. What you see in the sentence is possibly a misplaced modifier[1]. Supposedly, one could understand the phrase "Apart from Federer" to refer to the speaker ("I"), but in this sentence I think such a misunderstanding would be highly unlikely. One can find examples of far more egregiously misunderstandable sentences at the sources linked in the footnotes. Additional sources to understand misplaced modifiers are at [yourdictionary (dot) com][2], and [opentextbooks (dot) org][3].

The Walden Univ source describes misplaced modifiers as

When a modifier is ambiguously or illogically modifying a word, we consider it a misplaced modifier.

And, the yourdictionary description:

It’s important for modifiers to stick close to the word or words they’re modifying. When they stray too far, they become misplaced modifiers — and if they get too far, it may look like they’re modifying something else.

You will notice that none of the resources linked puts modifier placement in the "rules to obey" column, as we have with some other grammatical instances. What is important in each description is that the modifier be placed for clarity of understanding. A pedant might take issue with the placement of "Apart from Federer", because it is not directly adjacent to "a stronger player". However, since, in this usage the "apart from" phrase clearly modifies "a stronger player", it would not be misplaced. If one were concerned for absolute clarity, with no chance for misinterpretation, then perhaps:

I have not seen a stronger player, apart from Federer.

or

I have not seen, apart from Federer, a stronger player.

However, personally I find both cases to be more uncomfortable and lacking the immediacy and dynamic exclamatory style of the original. As I said above, one can easily find examples of truly misplaced modifiers, whose placement would lead to gross misunderstanding or confusion. But I don't think this is one.

  [1]: https://academicguides.waldenu.edu/writingcenter/grammar/modifiers
  [2]: https://examples.yourdictionary.com/examples-of-misplaced-modifiers.html
  [3]: https://www.opentextbooks.org.hk/ditatopic/4537

While I like Jeffrey Carney's answer (and upvoted it), I will write my own because I believe one thing about the question should be made clear. While the questioner's sentence may seem confusing or ambiguous to some, it is not grammatically incorrect. It would be clearly understood by native speakers under most circumstances.

However, as a matter of style, this sentence may not constructed in the clearest way. What you see in the sentence is possibly a misplaced modifier[1]. Supposedly, one could understand the phrase "Apart from Federer" to refer to the speaker ("I"), but in this sentence I think such a misunderstanding would be highly unlikely. One can find examples of far more egregiously misunderstandable sentences at the sources linked in the footnotes. Additional sources to understand misplaced modifiers are at [yourdictionary (dot) com][2], and [opentextbooks (dot) org][3].

The Walden Univ source describes misplaced modifiers as

When a modifier is ambiguously or illogically modifying a word, we consider it a misplaced modifier.

And, the yourdictionary description:

It’s important for modifiers to stick close to the word or words they’re modifying. When they stray too far, they become misplaced modifiers — and if they get too far, it may look like they’re modifying something else.

You will notice that none of the resources linked puts modifier placement in the "rules to obey" column, as we have with some other grammatical instances. What is important in each description is that the modifier be placed for clarity of understanding. A pedant might take issue with the placement of "Apart from Federer", because it is not directly adjacent to "a stronger player". However, since, in this usage the "apart from" phrase clearly modifies "a stronger player", it would not be misplaced. If one were concerned for absolute clarity, with no chance for misinterpretation, then perhaps:

I have not seen a stronger player, apart from Federer.

or

I have not seen, apart from Federer, a stronger player.

However, personally I find both cases to be more uncomfortable and lacking the immediacy and dynamic exclamatory style of the original. As I said above, one can easily find examples of truly misplaced modifiers, whose placement would lead to gross misunderstanding or confusion. But I don't think this is one.

  [1]: https://academicguides.waldenu.edu/writingcenter/grammar/modifiers
  [2]: https://examples.yourdictionary.com/examples-of-misplaced-modifiers.html
  [3]: https://www.opentextbooks.org.hk/ditatopic/4537