Boston’s office market continues to defy the odds, but the silver lining may be tarnishing.

Downtown Boston’s overall vacancy rate fell to 7 percent in the third quarter, down from 8.1 percent for the same period last year, ac-cording to a report by real estate brokerage firm Cushman & Wakefield. Rents swelled to $50.93, up from $45.14 last year – a 12.8 per-cent hike. For Class A space, the vacancy rate was flat at 6 percent, while rents increased 3.4 percent to $66.45, up from $64.22 a year ago.

But the latest numbers may not tell the whole story.

As layoffs explode on Wall Street and the ripple effect is felt in Boston’s financial services industry, brokers are bracing for a wave of office space to hit the market, followed by falling rents and a surge in vacancies. The third-quarter data comes on the heels of a report from the state’s Executive Office of Labor and Workforce Development which revealed financial sector employment is down 1,500 jobs from one year ago.

“I already see landlords bending on terms and rents that favor tenants because they’re nervous,” said Debra Stevens, principal at The Stevens Group, a commercial real estate firm in Boston.

Rents In The Rear-View

Leases that were signed earlier this year at Blackstone Group’s properties at One Post Office Square, 60 State St. and Rowes Wharf at $100 per square foot are a thing of the past, according to brokers. Last year, the private equity firm rocked the commercial real estate world when they paid $39 billion for the purchase of 22 buildings, 17 of them in downtown Boston, and vowed to boost rents.

“When Blackstone bought Equity’s properties they sought rents at $100, and in a few cases they got those rents for a few small spaces,” said William McCall, president of McCall & Almy, a Boston firm that exclusively represents tenants. “But no one is paying that now. In the last 60 days, deals were done in the $70 range.”

William Motley, managing director at Jones Lang LaSalle, acknowledged that landlords are cutting rents to get deals done. But he noted the amount of available sublease space is 839,000 square feet, or 1.4 percent of the downtown market total of 58 million square feet. With new office space still years away, he said, the laws of supply and demand will keep rents up.

Every downtown submarket experienced rent increases in the third quarter, according to Cushman & Wakefield. Among Class A prop-erties, the Financial District commanded the highest asking rents at $69.11 per square foot, up from $67.02 one year ago, a 3 percent hike. The Back Bay, the city’s other expensive office district, saw rents increase to $59.04, a nearly 4 percent boost over last year’s rate of $56.97.

Still, McCall, the tenant representative, predicted lower rents for the fourth quarter.

“I expect to close the year with either lower rents or more tenant incentives including free rent or tenant improvement,” he said.