In his first outing since a breakthrough run to reach the final of the Paris Masters in November last year, Janowicz was defeated 4-6, 7-6 (7/5), 6-4 after an erratic display.
Janowicz committed 51 unforced errors and served 10 double faults to hand the match to Baker.
The Pole took the first set comfortably, but lost concentration early in the second, finally rallying at 5-1 down in the third, when it was too late to stop Baker.
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