2004 - 2007.

Federer and Nadal are dominating the game -- Nadal on clay courts (a surface
that the French Open is unique in using among all four grand slams), and Federer
on everything else.
2008.

Djokovic wins the Australian. Nadal finally beats Federer at Wimbledon in what
will probably be remembered as the best tennis match of all time -- let's make a
note of this monumental occasion!
2009.

Nadal wins his first Australian Open. Federer finally wins the French Open!
Nadal disappointedly failed to defend his Wimbledon title, so made a note there
to highlight this. del Potro wins the US Open in epic fashion in a time when
Federer still looks to be on the top of his game.
2010.

Nadal owned this year! Made a special case for this year. Er, also a special
case for the special case for the Australian. The Wimbledon block unfortunately
needs to be reworked to keep the comment about 2009 specific to that year.
2011.

Djokovic owned this year! Better make a special case for him too. Except also a
special case to the special case... Nadal still won the French.
2012 - 2013.

Even though Federer won the 2012 Wimbledon against Murray, the London Olympics
that year was so soon afterwards that it's also worthy of our mention --
especially since it served as a rematch between the two, which Murray won. That
was immediately followed by Murray finally getting his maiden Slam from the US
Open that year, later followed by Wimbledon (for real!) in 2013. Meanwhile,
Nadal gets another US Open. Djokovic defends his Australian crown two more
times... in fact, since he looks so successful, let's invert the logic there to
favor him as the default winner. Has Federer reached his peak?
2012 - 2013. Improvement 1.

Dead code elimination: we already have all four Grand Slams covered.
+ Eliminated final redundant `else` clause.
2012 - 2013. Improvement 2.

Pulled up block for French Open, since Nadal is clearly dominating it, and the
Year of the Soderling was likely a fluke.
+ One fewer `else` clause from Djokovic's 2011 almost-sweep.
2012 - 2013. Improvement 3.

Refactored the winner-determining function into its own helper to take advantage
of early returns. This allows for changing the comment-only branches (such as
the 2009 Wimbledon commentary) to truly do nothing. Converted all `if` branches
so they must return, and now there's no more need of top-level `elif` uses. Also
added attempted explanations for each top-level `if` block -- because that tells
a much more interesting story about the modern men's tennis scene than a bunch
of nested `if` statements.
+ One fewer `else` clause for French Open (implied by return at end of block)
+ One fewer `else` clause for 2010 (implied by return at end of block)
+ One fewer `else` clause for Wimbledon (implied by return at end of block)
+ One fewer `else` clause for Australian Open (implied by return at end of block)
+ One fewer `else` clause for US Open (implied by return at end of block)
[RFC] 2012 - 2013. Improvement 4.

Possibly controversial claim -- assume that Federer will win anything that's not
covered by existing logic, because he is arguably the Greatest Tennis Player of
All Time. Now the first-level blocks don't always return immediately, because if
they don't match to a result from the inner branches, it's always Federer who
ends up the winner. We'll fully test out that hypothesis against future results.
+ French, 2010, Wimbledon and US Open all no longer return by default