Janzen, winner of the 1993 and 1998 titles, began feeling ill during a Thursday practice session, his agent Tommy Limbaugh said.
He visited the doctor for tests and was sent home Thursday night, but his condition worsened and he had surgery at Sand Lake Hospital on Friday morning.
Doctors advised against practicing for at least a week, Limbaugh said, and a timetable for a return to tour competition is uncertain. But Janzen should recover in time to play in the Open, which is set for June 16-19.
'How soon he can play prior to that is pretty much up to his body and how it responds,' Limbaugh said.
In 10 events this year, Janzen has two top-20 finishes while missing the cut six times. He ranks 133rd on the money list with $181,040.
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