He was unable to comment because he was on his way back from Japan. A spokeswoman said more detail on why his bid had been withdrawn would be announced tomorrow.
Reuters reported this morning that Mr Groser had been encouraged to withdraw after failing to gain enough support from the WTO's 159 members.
Diplomatic sources were quoted as saying the race had come down to Mexico's Herminio Blanco or Brazil's Roberto Azevedo.
In recent months he has travelled overseas extensively lobbying for support, racking up expenses of almost $260,000 in the first three months of the year.
That figure topped the travel expenses of Foreign Affairs Minister Murray McCully, who spent $130,000, and Mr Key, who spent $100,000.
Mr Key said Mr Groser's travel bill was money well-spent.
"If you look at what the WTO can achieve -they can achieve things that a bilateral or multilateral free trade agreement can't achieve, and that is subsidy reform.''
Mr Key said much larger sums were spent on New Zealander Mike Moore when he was in that position.