Apple Inc. named Senior Vice President Jeff Williams its chief operating officer, filling a role that has been vacant since Chief Executive Tim Cook assumed his post in 2011. Mr. Williams, who joined Apple in 1998 and is responsible for overseeing its massive supply chain and operations, has been expanding his role at the company, including overseeing the development of the Apple Watch. He's often referred to as "Tim Cook's Tim Cook," a nod to the important operations role that the Apple chief played for former CEO Steve Jobs before his death. In a statement, Mr. Cook said Mr. Williams is "hands-down the best operations executive I've ever worked with." Mr. Williams becomes the fourth C-level executive at Apple including Mr. Cook, chief financial officer Luca Maestri and chief design officer Jony Ive. A senior executive at the company, who declined to be identified, said the move isn't necessarily a sign that Mr. Williams is the heir apparent to Mr. Cook and that many members of the current executive team have what it takes to be the future CEO. The new title formalizes a role that Mr. Williams has been playing inside the company, the executive said. Apple also named senior vice president for marketing Phil Schiller to oversee the App Store, which now includes apps for the iPhone, iPhone, Mac, Apple Watch and Apple TV. This was a role filled by Apple's services head Eddy Cue, whose responsibilities have expanded in recent years with Apple's move into streaming music, mobile payments and, possibly, a streaming video service. The company also appointed Johny Srouji, who oversees the team that designs Apple's semiconductors, to the executive team with the title of senior vice president for hardware technologies. Apple also said it had hired Tor Myhren, the chief creative officer and president of ad agency Grey New York, to be a vice president of marketing communications, a role that is responsible for Apple's advertising campaigns. He plans to join the company in the first quarter of 2016. The new appointments come at a critical juncture for Apple. After riding a wave of strong iPhone sales from the release of bigger-display phones, Apple is facing questions once again about whether it can continue to grow. Analysts are forecasting that iPhone sales may decline in the current fiscal year, ending in September, after last year's blockbuster 37% sales increase. With so much of Apple's business weighted toward the iPhone, the company is pushing to find areas of growth with new hardware products such as the Apple Watch along with new services like Apple Music. Shares of Apple, the most valuable company in the world by market capitalization, are down 13% over the past six months. In early trading Thursday, Apple shares fell 14 cents, to $111.20.