Williams overwhelmed the seventh seed, 6-4, 6-3, slamming aces past Coetzer midway through the second set in a match the American dominated with power. But the second seed's erratic forehand at times threatened to undermine the chance to reach her third successive final in Germany.
"I was just going for a lot today," said Williams. "I didn't hold back. I took a lot of risks."
Williams was joined in the semi-finals by France's Mary Pierce, who defeated Conchita Martinez of Spain, 6-4, 6-4, in an hour and 24 minutes. Pierce, the fourth seed, converted her third match point when Martinez mis-hit a backhand.
Williams next faces the winner from Spain's Arantxa Sanchez Vicario and Italy's Silvia Farian, while Pierce plays either Jana Novotna, the top seed, or the Austrian Barbara Schett.
Russia's Yevgeny Kafelnikov, who takes over as world No 1 next week, has pulled out of the German Open starting on Monday because of a viral infection.