Mr Xenophon said he was considering taking legal action against Premier-elect Steven Marshall for having suggested he had done a deal with Labor.
Arriving to a rockstar welcome before his jubilant supporters shortly after, Mr Marshall thanked his predecessor as he spoke of a "new dawn" for the state.
Few pundits are sticking their necks out in an election featuring two evenly-matched major parties and the unique influence of Nick Xenophon's SA Best, who, by running an open ticket, have left pollsters guessing as to where preferences might go.
It follows a first ad released at the beginning of the campaign which featured Mr Xenophon calling on South Australians to vote for the party with a jingle and dance moves.
'So far some of the figures have been encouraging, ' he said.
Mr Weatherill, who has been Premier since 2011, visited Government House on Sunday morning to officially resign as premier, saying he planned to see out his term in State Parliament.
'I'm sorry I couldn't bring home another victory, but I do feel like one of those horses that has won four Melbourne Cups.
"We know that more renewable energy means cheaper power, and that's why we have increased our renewable energy target to 75 per cent and also introduced a new renewable storage target of 25 per cent", Weatherill told reporters in Adelaide on Friday.
'I wished him all the best on his endeavours to really take that role and make it his own'. "We can't turn back now".
Opposition Leader Steven Marshall's Liberal Party was projected to win 24 seats in the 47-seat parliament by ABC election analyst Antony Green.
"This is the single biggest issue facing South Australians", he said.
'Now, we have been given a wonderful opportunity by the people of South Australia'.
"I'm looking forward to that, being the state member for Cheltenham".
More than 1.2 million South Australians head to the polls on Saturday for one of the most unpredictable elections in the state's history.
They pointed to the swing against the Liberal Party and to Labor's apparent retention of potentially all of its heartland seats despite the party's extended period in office and despite the spirited challenge from the SA Best phenomenon.
The state has had a testy relationship with the centre-right Federal government ever since Canberra refused to bail out the now dead vehicle industry in 2014.
Prime Minister Malcolm Turnbull offered his congratulations to Mr Marshall.