Interesting discussion over at Literal Minded regarding the emergent usage of Grand Slam (in tennis journalism) to refer to a win in one leg of the four tournaments that make up the Grand Slam of tennis, instead of referring to a win in all four legs. Neal succinctly sums it up as a case of a compound losing its head.
I'd assumed the phrase Grand Slam was extended from baseball, where it refers to a home run with the bases loaded (thus scoring 4 - hence a shared notion of fourness). However, several commenters on Neal's post point to an earlier usage in bridge (taking all thirteen cards in one trick), which OED supports with citations older than the appearance of modern baseball. This means I have to add an amendment to the old post about the use of Grand Slam in different games.
Oxford's earliest baseball citation for Grand Slam is 1953, so it's a mystery where the baseball usage fits in relative to the bridge and tennis usages. Right now I have no way of determining the age of the tennis Grand Slam usage, but I suspect it may have developed by way of baseball. Here's why: in bridge, it invokes "all at once" (i.e., allness), and in tennis, it invokes "four important tournaments" (i.e., fourness). In its baseball usage, it invokes both: allness (in terms of runners on base, or highest score on one hit) and fourness (number of runs scored on a grand slam).
This is not the only example of a metaphor crossing from cards to different sports and with different usages; check out alternate usages of ace as "the best" and "score with one swing".