Google Inc. headed toward its biggest quarterly rally in seven years—up 30% through Thursday—as concerns faded about its Motorola acquisition and the price of mobile-search ads.

The search company's third-quarter climb is nearly five times the 6.2% gain in the Standard & Poor's 500-stock index over that period. The advance has Google within striking distance of unseating Microsoft Corp. as the third-largest U.S. company by market value. Friday morning, Google shares were down $1.78, or 0.2%, to $754.72.

Google's...