DUESSELDORF, Germany -- Ivo Karlovic of Croatia advanced to the Duesseldorf Open semifinals on Thursday, hitting 19 aces in a 6-4, 3-6, 6-3 win over defending champion Juan Monaco of Argentina.
The seventh-seeded Karlovic will meet Jiri Vesely of the Czech Republic on Friday. Vesely spoiled Jurgen Melzer's 33rd birthday by beating the Austrian 6-1, 6-1 to advance to his first ATP semifinal.
Melzer is playing again after missing seven months with a shoulder injury.
Denis Istomin of Russia upset third-seeded Andreas Seppi of Italy 6-3, 6-4 to record his 150th career win.
The match between top-seeded Philipp Kohlschreiber of Germany and qualifier Mate Delic of Croatia was interrupted twice by rain and finally abandoned because of darkness, with Kohlschreiber leading 7-5, 4-4. Delic is in his first tour-level event.