Andy Murray and Jurgen Melzer.
This should be a classic lefty vs. righty matchup. Murray has the advantage as he has taken all four meetings against the Austrian, Melzer. They have played close matches though, remember back to the 2008 U.S. Open and the 5-setter they played.
Murray hasn't been tested to this point in Melbourne. Melzer will give him that test. Melzer's left-handed serve will keep Murray off-balance on the returns, and Melzer's aggressive play could give Murray fits.
Still, Murray appears in great shape and his mental state is also good. Look for Murray to pull this out in four tough sets, including a couple tie-breaks.
A good battle between these tough competitors.