Apple's rebound makes it a slight favorite to be the world's largest company by year-end

Traders now see Apple as a slight favorite to be the world's largest company by market value when the year ends, a sharp change from a day earlier, when the iPhone maker looked unlikely to hold the top spot. The question is a two-way race with Nvidia, and Nvidia was the favorite when this story began.

The likely reason is a swap in the companies' stock-market rankings. Apple shares rebounded on Wednesday while Nvidia shares fell, reversing the two companies' market-value positions shortly after the U.S. stock market opened, according to Stock Analysis and 24/7 Wall St. That made an Apple lead at year-end more plausible, though the ranking can change again before Dec. 31.

The move was large. Bets on Apple sat in unlikely territory through Tuesday and crept up overnight. They climbed through Wednesday morning and jumped to a toss-up once U.S. trading began, a surge of nearly twenty points. Bets on Nvidia fell the other way, from likely to a toss-up. Across the contest, Apple is now narrowly ahead, with Nvidia close behind.

The lead has been changing hands. Apple finished ahead on Oct. 30 as Nvidia fell in a semiconductor selloff. Nvidia took the title back at Tuesday's close, when Apple fell 1.9% and Nvidia rose 2.29%, 24/7 Wall St. reported. Bets on Apple swung widely in October as well, jumping on Oct. 22 and Oct. 29 and slipping in between.

Apple reported fiscal fourth-quarter revenue of $114.5 billion on Monday, up 12% from a year earlier. It also warned that memory costs would weigh on margins in the holiday quarter, which helped send its shares lower the next day, according to the company's announcement and reporting.

In early trading Wednesday, a real-time quote feed put Apple at $348.36, up about 5%, and Nvidia at $204.19, down about 1.2%. That implied market values of roughly $5.12 trillion for Apple and $4.95 trillion for Nvidia. Those are intraday estimates, not closing figures, and other trackers have published different numbers, so the exact gap is uncertain.

The question turns on which company is largest at the market close on Dec. 31, not on who leads in the meantime. The next test is Nvidia's earnings report on Nov. 17, when it is due to release third-quarter results followed by a conference call. Its results and guidance could shift how investors value the two companies against each other.