Anwar was sentenced to five years in prison in March on charges of sodomizing a male aide after the appeals court overturned an earlier acquittal. The Federal Court will hear his appeal Oct. 28-29.
The United States and international human rights groups call the legal moves against Anwar politically motivated.
"It's a foregone conclusion that there will be a conviction unless there is a miracle," Anwar said late Tuesday at a dinner with the foreign media.
Anwar, 67, was accused of sodomizing a male aide in 2008, but acquitted by the High Court in 2012. He has said the charges were trumped up to kill his political career.
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He is the most potent threat to Prime Minister Najib Razak, whose party has ruled Malaysia since independence in 1957 but faces declining support.
Anwar previously was imprisoned for six years after being ousted as deputy prime minister in 1998 on earlier charges of sodomizing his former family driver and abusing his power. He was freed in 2004 after Malaysia's top court quashed that sodomy conviction.
"It's not that I enjoy prison. I dislike it immensely," he said. "But I have no option. This is my homeland. I have vowed to fight from this country. I will continue my struggle."
He said it would be toughest on his family and that he had apologized to them profusely.
Anwar said he can't predict if jailing him would spark massive street protests like in 1998, but believes it would galvanize further support for the opposition.
He led a three-party alliance to unprecedented gains in 2008 elections and made further inroads in 2013 polls. Najib's National Front coalition won with a slimmer majority and lost the popular vote to the opposition.
Sodomy, even consensual, is a crime in Muslim-majority Malaysia and is punishable by up to 20 years in prison.