DUSSELDORF, Germany—Ivo Karlovic advanced to the semifinals of the Dusseldorf Open on Thursday, hitting 19 aces in a 6-4, 3-6, 6-3 win over defending champion Juan Monaco.
The seventh-seeded Karlovic will meet Jiri Vesely in Friday's semifinals. Vesely spoiled Jurgen Melzer's 33rd birthday by winning 6-1, 6-1 to advance to the first semifinal of his career.
Melzer is playing again after missing seven months with a shoulder injury.
Denis Istomin defeated third-seeded Andreas Seppi, 6-3, 6-4. Istomin will play either top-seeded Philipp Kohlschreiber or qualifier Mate Delic for a spot in the final.