Christie’s auction house reported its highest-ever sales total for the first-half, topping $4 billion during the first six months of this year and highlighting the continued strength of the global art market.

Total sales were up 35 percent in the first six months of 2018, according to Christie’s, which is privately held. More than three-quarters of the gain came from the sale of the David and Peggy Rockefeller art collection, which totaled $835 million and set a new record for a single collection.

More than 85,000 people viewed the items from the collection during the public exhibits in Asia, Europe and the U.S., and bidders flocked from around the world in May to buy some of the legendary Rockefeller family's 2,000 pieces of art.

The sale also generated new customers for the world's leading fine art auction house. The new clients who initially came in for the Rockefeller sale accounted for about 15 percent of sales during Christie's subsequent 20th Century week, Christie’s said.

“We were really able to capture the engagement and the opportunities that came from Rockefeller,” said Marc Porter, chairman of the Americas for Christie’s. “It allowed us to create a deeper conversation with those clients.”

The bulk of Christie’s business is auctions, with auction sales up 28 percent to $3.6 billion. But its private sales, selling directly to a client, more than doubled to $390 million, and its online-only sales were up 50 percent to $38 million.

Christie’s said the outlook for the rest of the year also looks to encouraging, and “the current collection and consignment activity remains particularly strong.”

Porter said the early signs for the fall sales look especially promising. While negotiations for big consignments —items offered to Christie's for sale — for the November sales in New York typically start in August and September, this year the discussions started in June and July.

“This summer was a marked change from the past,” he said. “We are talking about some very interesting things “ for the fall, including top works from some major estates, private collections and foundations.

Christie’s also continued to dominate the very top of the art market. Fully 80 percent of the highest-selling pieces of art sold across the world in the first half of 2018 were sold by Christie's, with 55 lots selling for more than $10 million, up from 38 last year.

Despite all the attention on Chinese buyers in recent months, Americans continue to be the main driver of the art market boom. U.S. buyers accounted for 45 percent of total purchases at Christie's in the first half — a big jump from the usual share of about a third. Asian buyers accounted for 24 percent and buyers from Europe, the Middle East, Russia and India accounted for 31 percent.

Christie’s online business is also growing as more collectors buy art and collectibles through their smart phones or laptops. Bidders spent $120 million in online-bidding and online sales in the first six months of the year. Online sales also helped attract new buyers: 27 percent of which during the first half of the year were new to Christie's.

Christie’s is owned by Groupe Artemis, the holding company of French luxury-goods magnate Francois-Henri Pinault. Because it releases only selective information, its profits, earnings and balance sheet information are not disclosed.