By Sarah Nassauer 

For the first time in nearly a decade, online retail sales over the Thanksgiving holiday weekend didn't increase from the prior year, according to some industry estimates.

The data, while preliminary and up against last year when the pandemic sent a wave of spending online, shows shoppers embraced stores on Black Friday and supply-chain concerns pulled some purchases earlier.

This year online Black Friday sales fell to $8.9 billion from $9 billion last year, according to Adobe Inc., while Thanksgiving Day online sales were roughly flat at $5.1 billion, the first time sales didn't increase since Adobe started tracking the figures in 2012. Thanksgiving weekend shopping had for years shifted online at the expense of in-store shopping.

While visits to bricks-and-mortar locations rebounded, they remain below pre-pandemic levels, a sign that shoppers are back but that a wider retail shift toward buying online is still intact. RetailNext, a firm that tracks shopper counts in thousands of stores with cameras and sensors, said store traffic rose 61% this Black Friday compared with last year but was down 27% from 2019. Sensormatic Solutions, another firm that tracks store traffic, said Black Friday traffic rose 48% from last year, but was 28% lower than in 2019.

"Part of that is attributed to Covid fatigue," said Brian Field, a senior director at Sensormatic. "Part of that is that there is a continued push earlier in the season for shopping," he said.

Ginny Rogers, a 57-year-old operator of a car-repair shop, said she did all her Christmas shopping online last year to avoid stores during the pandemic. This year she returned to the Walmart where she typically shops on Black Friday, near her home in Cairo, N.Y.

"I'm tired of not going out," said Ms. Rogers, piling her cart with sheets, socks and other sale items she likes to give as gifts. "It's nice to be back."

Overall sales on Black Friday were robust as expected by many industry consultants and executives. As of midafternoon Friday sales rose 29.8% compared with the same period last year, with sales in stores rising 42.9%, according to Mastercard SpendingPulse, which tracks sales activity online and in stores within the Mastercard payments network, combined with estimates for all other forms of payment, including cash.

"It speaks to the strength of the consumer," said Steve Sadove, senior adviser for Mastercard and former chief executive of Saks Inc.

The National Retail Federation, a trade group, expects U.S. retail sales during November and December to rise by a record 8.5% to 10.5% from a year ago to as much as $859 billion. The average increase was 4.4% over the past five years. The forecast was made before the World Health Organization declared Friday that a new strain of the coronavirus, first detected in southern Africa, was a global "variant of concern." The move triggered worries that travel restrictions and other curbs would spoil the global economy's recovery.

Overall online shopping during the holiday season has been spread over a longer period than in past years.

Shipping delays and a year and a half of pandemic-related product shortages nudged customers to buy early, retail executives have said in recent weeks. Sixty-one percent of shoppers surveyed by the NRF said they had started their holiday shopping before Thanksgiving, up from 51% a decade ago.

More consumers are feeling flush this year than last, benefiting from a strong labor market. Household spending rose 1.3% in October from a month earlier, while personal income increased 0.5% last month, the Commerce Department said last week. Consumers are spending at a faster pace than inflation, which recently hit a three-decade high.

Michelle Jamal, a nurse from Yonkers, N.Y., cut back on holiday shopping last year after pausing work because of Covid-19 safety concerns. This year, she plans to buy gifts for 10 to 15 people, both online and in stores, she said Friday at the Cross County Shopping Center in Yonkers. "It feels good to be out here," she said.

Surging demand for products, along with Covid-19-related manufacturing slowdowns and transportation constraints, has strained supplies of certain goods. Many large retailers such as Walmart Inc. and Target Corp. were able to bring in inventory ahead of the holidays, and executives have said they are well-stocked for the season in recent weeks. Some smaller chains, including Gap Inc. and Victoria's Secret & Co., are scrambling to keep their shelves stocked. Last Tuesday, Gap predicted supply-chain disruptions would result in about $550 million to $650 million in lost sales.

Some shoppers expecting to find many items out of stock over the weekend were surprised to find full store shelves.

"We went to Best Buy, and they have everything, it was the most I'd ever seen," said Lila Leu, 29, of Yonkers, who was shopping at a mall with her mom, Stacey Breindel, on Friday morning.

Monica Adames said it was her first time shopping on Black Friday, but she wasn't yet buying much because she couldn't find what she wanted as she browsed stores at The Shops at Atlas Park, an outdoor mall in Glendale, N.Y.

She wasn't able to find the hoodies she plans to give her nephews, said the 50-year-old who lives in the Bronx borough of New York City. "You would think it would be easy to find," she said. "I've been to several stores over a couple of weeks and nope. They have the ones with the half zipper, but not the whole zipper."

Overall, the early start to the season had many shoppers buying more, as retailers hoped by kicking off the season early.

"Walmart had Black Friday in the beginning of November, so I got most gifts then, online," said Alyson Boice, as she walked through the Walmart store near her home in Palenville, N.Y., early Friday morning. "I came grocery shopping, but now I'm buying," said the 39-year-old, her cart filled with craft sets and other toys.

"I was like, darn it!" she said, "I'm spending more because Black Friday is throughout the month."

Sharon Terlep and Charity L. Scott contributed to this article.

Write to Sarah Nassauer at sarah.nassauer@wsj.com