Dusseldorf - Ivo Karlovic of Croatia has advanced to the Dusseldorf Open semi-finals 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 semi-final.
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.
Results on Thursday from the ATP Dusseldorf tournament (x denotes seeding):
Jir Vesel (CZE) bt Jurgen Melzer (AUT) 6-1, 6-1
Denis Istomin (UZB) bt Andreas Seppi (ITA x3) 6-3, 6-4
Ivo Karlovic (CRO x7) bt Juan Monaco (ARG) 6-4, 3-6, 6-3