Plaid Cymru's Helen Mary Jones has overturned a Labour majority of just 21 to win back the Llanelli assembly seat she lost four years ago.
Helen Mary Jones decided to concentrate on winning back Llanelli
Ms Jones beat Labour opponent Catherine Thomas - the former AM - by 3,884 votes against a turnout of 49.39 per cent.
Elsewhere in west Wales, the Tories won both Carmarthen West and South Pembrokeshire and Preseli Pembrokeshire previously held by Labour.
Labour did maintain its domination in Swansea and Neath Port Talbot.
Ms Jones' last seat was via the regional list, but a rule change meant she had to gamble between standing for the constituency or the list in 2007.
The gamble paid off, with Labour saying its vote had "slipped away altogether".
Ms Jones said she had never doubted her decision to fight the Llanelli seat but, shortly after the declaration, she admitted her convincing win "had not really sunk in yet".
"I'm obviously thrilled. It's amazing, amazing," she said.
"Llanelli is my home community. I knew I had done the right thing when I made that choice," she told BBC Wales.
"Clearly I did not expect to have that decision validated to this extent - I'm very proud and quite humble."
She said she would fight for the health service which has been at the centre of local campaigning because of Labour's plans to reorganise hospitals, and for jobs.
Plaid had run a strong campaign to win the most marginal seat, which was one of the five key seats it lost in 2003.
Admitting defeat before the official result was declared, Labour's Llanelli MP Nia Griffiths said voters had "ganged up on what they see as the best option to get rid of Labour".
In Preseli Pembrokeshire, bank manager Paul Davies won the seat for the Conservatives, ousting Labour's Tamsin Dunwoody.
He polled 11,086 votes compared to Ms Dunwoody's 7,881 - a swing from Labour to the Conservatives of 8.6%.
"I'm absolutely delighted that the people of Preseli Pembrokeshire have given me the opportunity to represent them for the next four years," he said.
"Two years ago, the people of Preseli Pembrokeshire rejected Tony Blair's Labour Party and voted for the Conservatives for strong representation at Westminster.
"Today they have rejected Rhodri Morgan's Labour Party to ensure they get strong representation in the assembly."
The Tories also won a closely fought three-way fight in neighbouring Carmarthen West and South Pembrokeshire.
Conservative Angela Burns took 8,590 votes compared to Labour's Christine Gwyther, who had been the sitting AM, with 8,492 and Plaid Cymru's John Dixon on 8,340.