The board of German sportswear company Adidas (ADSGn.DE) has launched a formal search for a successor to long-serving Chief Executive Herbert Hainer, who has faced calls from some investors to step down after his contract was extended to 2017.

Hainer, in the job since 2001 and the longest-serving CEO of a German blue-chip company, wrote to staff confirming the move after monthly magazine Manager Magazin reported the world's second-biggest sportswear group was looking for his successor.

"The supervisory board will now start to look for the best possible candidate for my succession both inside and outside the Adidas group," he wrote. "This will be a long-term process that has just begun."

The company did not say whether Hainer would see out his tenure to 2017.

Adidas shares were up 4.4 percent by 0959 GMT (04:59 a.m. EST), making them the biggest gainers on the German index .GDAXI.

Hainer has come under pressure as Adidas lost market share to its bigger U.S. rival Nike (NKE.N), which has been better at creating a buzz with its brands among consumers.

Last month, Adidas reported a better-than-expected 2 percent rise in 2014 sales to 14.8 billion euros ($16.9 billion), helped by the 2014 soccer World Cup and the victory of the German team it sponsors. It is due to present full results on March 5.

In the letter to staff, Hainer said 2015 had got off to a "fantastic start" after moves to sell the non-core Rockport brand, restructure its struggling golf business and boost its brands with deals with singers Pharrell Williams and Kanye West.

Hainer also noted he had already promoted a raft of younger managers under new global brand chief Eric Liedtke and sales boss Roland Auschel, with 12 of their 26 direct reports new in their positions.

Forced to ditch ambitious targets set for 2015 - including a sales goal of 17 billion euros - Liedtke and Auschel are working with Hainer on a new five-year strategy due to be presented on March 26.

Manager Magazin said Adidas would set a target to reach 20 billion euros of annual sales by 2020, and an operating margin of at least 10 percent. Adidas declined to comment on the report.