Scheduled operations were postponed and hospital waiting lists allowed to grow as the Welsh NHS struggled to cut costs in the last year, says a new report.
Managers balanced the books but need to save £404m this year, said Wales Audit Office (WAO).
Health Minister Mark Drakeford said the NHS "remains resilient" despite challenging circumstances.
But political opponents said the watchdog's report showed the health service was under "colossal" strain.
The WAO report indicates only £192m of the NHS budget cuts needed have been identified which leaves a potential gap of £212m.
It also suggests it is becoming "tougher" to find savings that do not impact on patient care and safety.
Some health boards, which manage hospitals, achieved their savings by reducing the number of planned operations on patients at the end of the financial year to help manage budget pressures.
According to the report published on Tuesday "reducing activity in this way is poor value for money and not an efficient and sustainable approach" to make savings.
It also says "several local health boards have taken difficult decisions to allow performance against waiting times to slip" to help balance their books.
Auditor General for Wales, Huw Vaughan Thomas said the Welsh NHS had worked hard to achieve financial break-even in 2012-13 but "breaking even is a small part of the story".
"Service performance on some key patient-focused areas has worsened," he said.
"Waiting times for planned treatments have deteriorated over the past three years, with a growing number of patients waiting more than six months for their treatment.
"Performance in emergency care has also fallen although the reasons for this are complex - emergency departments are increasingly stretched meaning patients are waiting longer to be treated or admitted than in the past three years."
Shadow health minister Darren Millar, Conservative AM for Clwyd West, said: "Performance in the Welsh NHS has nosedived and patient care has been compromised as a result of Labour's record-breaking budget cuts.
"It is clear from this report that while health boards may have broken even on paper, services have been damaged and patients have suffered as a direct result."
Liberal Democrat AM for North Wales, Aled Roberts, who is a member of the Public Accounts Committee, said: "The Welsh NHS is under a colossal amount of strain.
"Wales has the longest ambulance waiting times in any part of mainland UK, cancer waiting times not met since 2008 and A&E targets that have never been met.
"To think some of our NHS services could continue to deteriorate further is shocking."
Last year the NHS in Wales, overall, succeeded spending less than it received over the financial year which the report says was "commendable".
'Bleak financial position'
But this was largely as a result of a cash-injection of £92m given to health boards at the end of last year by Welsh government.
The WAO reveals that the extra cash was diverted from from other areas within the health budget.
It said that providing the extra financial support during the financial year gave "mixed messages" to NHS bodies after the Welsh government previously insisted that no "bail-outs" would be provided to struggling health boards.
Dr Keshav Singhal, a consultant orthopaedic surgeon at the Princess of Wales Hospital in Bridgend, said the report came after the NHS had faced an unprecedented level of emergency admissions over the winter, leaving hospitals with too few beds to cope.
He said: "In the quest for savings, most of the health boards have cut down the number of beds to a bare minimum so there's no slack in the system.
"We're running at near 100% capacity all the time. So while that's good for savings and good for efficiency, it's not good to absorb some unexpected pressures."
But Dr Singhal told BBC Radio Wales he was "optimistic" about the prospect of achieving more savings in the future.
He said: "There's a lot of further efficiencies we as clinicians can make. All the clinicians have to take it upon themselves to find further efficiencies and improvements in quality which in turn save money.
"But at the same time, there has to be some flexibility in case we have to open an extra ward in winter because of unexpected pressures."
Health Minister Mark Drakeford said the WAO report recognised "what it describes as the bleak financial position facing Welsh public services".
"Despite this very difficult set of circumstances the report outlines the Welsh NHS remains remarkably resilient.
"Even in challenging times, performance can improve," he said.