Starbucks Corp. (SBUX) - Get Report shares opened lower Friday even after the world's biggest coffee chain posted stronger-than-expected second quarter earnings, and boosted its full-year outlook, thanks to surprisingly solid same-store sales at home and market share gains in China.

Starbucks said adjusted earnings for the three months ending in March, the company's fiscal second quarter, rose 13% from the same period last year to 60 cents a share, four cents ahead of the Street consensus. Group sales were also firmly higher, rising 5% to $6.3 billion and essentially matching analysts' forecast.

Comparable store sales in the U.S. rose 4% from last year, Starbucks said, as a re-tooled menu brought in more customers in the afternoon period and a delivery partnership with Uber Eats boosted activity around the country. Same-stores sales in China were also impressive, rising nearly 3% despite a significant challenge from rival start-up Luckin Coffee, as a delivery partnership with Alibaba Holiding Co. (BABA) - Get Report bore fruit and more than 1 million people joined its rewards program. 

"The strength of our performance in Q2 has further validated our Growth at Scale agenda and the strategies we are pursuing to create long-term shareholder value with more sustainable, predictable business results, driven by focused and disciplined execution," CEO Kevin Johnson told investors on a conference call late Thursday . "We're making solid progress on each of our key strategies. But strategy is ultimately about execution."

"With greater focus and discipline, we have positioned our Company for the next chapter of growth, growth that is anchored in our mission, our values and in our brand promise," he added. "We are playing the long game as we continue to look to the future and build an enduring company."

Starbucks shares were marked 1.26% lower at the start of trading Friday to change hands at $76.21 each, paring gains from the $77.23 record high the stock reached yesterday and a move that would trim its year-to-date gain to around 18.3%.

Looking into 2019, Starbucks said it now sees full-year adjusted profits in the region of $2.75 and $2.79 per share, a six cent improvement from its prior forecast and well ahead of the Refinitiv-collected estimate of $2.71 per share.

One reason for the early market reaction to last night's earnings could be related to concerns over the sustainability of store traffic, as well as consistent profit margin growth. 

"These dynamics put in context that there is still uncertainty about sustainability and Starbucks' appropriate growth potential over time that keeps us on the sidelines," said BMO Capital Markets analyst Andrew Strelzik, who boosted his price target on the stock to $70 a share. "These issues could return to the forefront if (comparable store sales) begin to retreat as Starbucks laps tougher compares as early as 4Q, especially given resilient valuation as Starbucks' long-term growth rates have moderated over the last 2-3 years."