SAN FRANCISCO — As the digital advertising market booms and demand for smartphones wanes, Alphabet could soon dethrone Apple as the world’s most valuable company.

If it happens, Alphabet will move to the head of the class just five months after Google reorganized itself under the holding company.

The Silicon Valley rivals could trade places as early as today, given how rapidly the financial gap between them is narrowing. At the end of trading on Thursday, Apple’s market value stood at $522 billion; Alphabet was worth $515 billion.

That’s a dramatic swing from where things stood just 13 months ago. Apple then boasted a market value of $643 billion, almost twice Google’s $361 billion.

Since then, investors have soured on Apple. The company has struggled to come up with another trend-setting product amid slumping sales of its most important device — the nearly nine-year-old iPhone, which accounts for roughly two-thirds of Apple’s overall sales.

Apple has already acknowledged the iPhone will begin this year with its first quarterly sales decline since it debuted in 2007. The slowdown helped push down Apple’s stock price by 15 percent since the end of 2014.

In contrast, Google has maintained its leadership in the lucrative Internet search and ad market while building other popular products in video, mobile, Web browsing, e-mail and mapping. That bundle of services brings in most of Alphabet’s revenue, and is expected to deliver growth in the 15 percent to 20 percent range as marketers shift more of their budgets to digital services.

Alphabet also has impressed investors by reining in spending. Google hired Wall Street veteran Ruth Porat as its CFO.