This is a humbling day for Microsoft.

The software giant, once the dominant force in computing, watched as Google whizzed past it to become the second-most valuable tech company, behind Apple.

Google caught up to its longtime rival right after the opening bell. Google is up about a percentage point at $761.13, valuing it at $248.8 billion. Microsoft is down slightly at $29.60, giving it a market cap of $248.1 billion.

Although the two could change places before the end of the day, Google’s shares have been skyrocketing as the 14-year-old search powerhouse extends its reach.

Google’s ad business is booming, and the company is No. 1 in both search and mobile advertising. It also is on pace to supplant Facebook as the dominant player in display advertising.

Goolge’s Android mobile operating software also commands the greatest market share in the smartphone space.

Meanwhile, Microsoft is trying to reverse its fortunes as the world shifts away from its software-centric model to an online-centric one. Its mobile strategy includes its new Windows 8 software, which works on tablets, smartphones and PCs.

gsloane@nypost.com