Roddick, the tournament's top seed, was hampered by a back ailment, which started in his quarterfinal win over Taylor Dent on Friday and twice required medical treatment on Saturday. In the quarterfinal match, he was able to escape from two match points.
Ljubicic took full advantage, needing only 64 minutes to advance to his second ATP Masters Series final this year. He was beaten by Rafael Nadal in Madrid two weeks ago.
Ljubicic has won 20 of his last 22 matches, and is 15-8 in Masters Series tournaments this year, 55-20 overall. He won titles at Vienna and Metz in recent weeks.
Ljubicic has won his last two meetings against Roddick and is 3-5 lifetime. He beat Roddick in a Davis Cup match earlier this year.
Elsewhere, Tomas Berdych beat Radek Stepanek, 6-1, 3-6, 6-3.