The lead being developed by TCS, the number one player in the Indian IT industry, and Cognizant, a close runner-up, has been in the making for more than five years.

From 2010 to 2014, TCS has added $1.5-2 billion in incremental revenue, while Cognizant added $1-1.5 billion. Incremental revenue, or the amount of additional revenue an IT company pulls in a year, is a key indicator of competitive strength because it includes the wins of new contracts and logos.

Third-placed Infosys added over $1 billion in 2010 and 2011, but dropped to much below $1 billion in next three years. In 2014, fifth-rung player HCL Technologies added more new revenue than either Infosys or Wipro.

"The performance gap has been there for the last five years. The gap could narrow but a catch-up would be hard," said an analyst with a Mumbai brokerage. He declined to be identified.

Analysts attribute the increasing gap to changes in the industry — moving from a time when headcount was a constraint to growth to a time when demand from clients is the constraint.

"As contracts go from time and material to fixed price, TCS has an advantage because of the efficient delivery engine that they have built.Cognizant is also very aggressive on investing in the sales side, which is why they are also growing," Sagar Rastogi, analyst with Ambit Capital, told ET.

Cognizant keeps its margins in the 19-20% band, compared with the rest of the Indian IT industry that targets above 24% margins. Of the remaining top five players, HCL Technologies has seen the strongest growth in most of its markets. It has added more revenue than Wipro in North America for the last five years and more revenue in Europe for the last three years. If fourth-ranked fourth-ranked Wipro does not catch up, it could fall further down the table.

"Assuming there are no acquisitions, we could see Wipro drop below HCL Technologies after FY17, they could even fall below Tech Mahindra," Rastogi, said. He added that HCL Technologies was seeing the benefit of strong demand from infrastructure services.

The divergence in performance between the top players is even starker when you drill down into individual markets.