The disproportionate contribution of gains from those five issues underscores the tech sector's eye-popping performance this year. Information technology has risen nearly 20 percent in 2017. By comparison, the second-best-performing sector this year — consumer discretionary — is up 11.5 percent in the period.

Approximately a third of the stock market's gains this year have come from just five stocks as of last week: Apple , Amazon , Facebook , Microsoft and Alphabet Class A shares, according to data from Howard Silverblatt, senior index analyst at S&P Dow Jones Indices. On Tuesday, Amazon broke above the major milestone of $1,000 a share for the first time .

Large-cap tech stocks have been the stalwart of the U.S. equity market this year, but other stocks need to pick up some slack to keep the good times rolling.

U.S. stocks have been on fire this year, with the S&P 500 up 7.7 percent but the tech-heavy Nasdaq up more than twice that amount with a 15.3 percent gain. The two indexes also notched record closing highs on Friday.

"The good news is the market is at an all-time high. The bad news is it happened on the back of a few tech stocks," said Art Hogan, chief market strategist at Wunderlich Securities. "One of two things happen from here: Either the leadership fails and drags the market lower, or we see other sectors pick up some slack."

Marc Chaikin, CEO of Chaikin Analytics, said financials could be one of those sectors. "Based on what we heard from the [Federal Reserve] last week, I think they're due for a second wind," he said.

The Fed has forecast two more interest-rate hikes this year, and last week discussed plans to unwind its $4.5 trillion balance sheet. The plan, released in the minutes of its last meeting, showed the Fed will slowly reduce the balance sheet in a deliberate process that includes announcing cap limits on how much it will trim its holdings in Treasury and mortgage securities.

Financials are essentially flat for 2017 but surged right after President Donald Trump's election in November on hopes of tax reform and deregulation. The S&P financial sector is up 0.7 percent so far this year.

Other stocks in the S&P "have to catch up. Otherwise, it leaves the market vulnerable to a correction," Chaikin said.