Despite the ever-increasing competition between Google and Apple, iOS remains an exceptionally important advertising platform for the search giant, with one Wall Street estimate attributing as much as three-quarters of Google's mobile ad revenue to users of Apple's devices.

Of the $11.8 billion in mobile search revenue Google booked in 2014, 75 percent — Â nearly $9 billion — Â came from iOS, according to a recent Goldman Sachs analysis cited by the New York Times. Half of that total is chalked up to a deal with Apple that makes Google the default search engine for mobile Safari.

That arrangement is thought to cost Google between $1 billion and $2 billion each year, and many believe that it will end sooner than later. Apple is rumored to be considering a switch to Yahoo or Bing, and might also enter the market with its own solution.

Apple is known to be working on a large-scale web search program, led by the team acquired with social analytics firm Topsy in 2013.

These developments put Google in a precarious position when it comes to mobile search, and losing iOS is a potentially disastrous scenario. Google has already seen mobile search volume eclipse desktop search volume in 10 markets — Â including the U.S. and Japan — Â and many new Android-based manufacturers have eschewed Google's own services in favor of other local options.