Kirsty Williams said party membership in Wales has grown since last year's general election.
Speaking on BBC's The Wales Report programme, she said her party had returned a better result than expected in assembly elections five years ago.
She also refused to rule out a deal with other parties this year.
"Since the general election, which was a very, very difficult set of results for us, our party membership in Wales has grown," she explained.
"We have record numbers of people out on the streets delivering campaigns."
Speaking about the last assembly elections, she said: "We defied expectations, we returned with a strong Liberal Democrat group that has punched above its weight for the last five years.
"I'm sure we can defy expectations again."
She also insisted the Welsh Lib Dems' record proved they can deliver on their promises.
Later in the interview, Ms Williams said she would not rule out a deal with any other party after the election to achieve her objectives but she was quick to point out that collaboration with UKIP was unlikely, as their values were "so far apart".
Last week, informal talks between the Liberal Democrats, Greens and Plaid Cymru for an assembly electoral pact failed to reach an agreement.
When asked if it was her party that pulled the plug on the talks, Ms Williams said: "We were approached informally by another political party about a proposition of doing a pre-election pact.
"We didn't think it was the right way to go."
Ms Williams added: "What keeps me going is that I know there are no political parties in this coming election that are committed to ensuring that there are the right number of nurses in our hospitals.
"We had to drag the Labour party kicking and screaming to introduce a system that put money into the education of our poorest children, and its working, its making a difference."
The Welsh Liberal Democrats can "defy expectations" in the forthcoming assembly elections, the party's leader has insisted.