NEW YORK (CNN/Money) - Even as he endures a major-victory drought on the golf course, Tiger Woods is still on top of his game: the endorsement game, that is.

Experts in the field say that it would take a far more prolonged and severe slump on the course to start eroding his value to advertisers and sponsors.

But even Woods' gold-plated image isn't bullet proof. Eventually, he'll have to start winning the big tournaments again if he wants to maintain his huge endorsement lead.

Woods goes into this weekend's U.S. Open not having won a major in two years -- since his 2002 U.S. Open.

Granted, most golfers would be thrilled to have his record since then -- eight wins and 18 other top ten finishes. But Woods has won eight majors in his career, including four in a row in 2000 and 2001. So many fans see his recent results as a major disappointment.

On Thursday, he started the U.S. Open with a 2-over par that put him six strokes behind the leaders, and he was being written off a contender this weekend by some of the prognosticators.

Still, his most lucrative sponsor, Nike, says it's not concerned yet, and it doesn't anticipate being concerned for quite some time.

Nike says Woods, shown here in his latest Nike ad, is as valuable to the company today as when he was on the top of his golf game.

"Do we think he has the same value he did two or three years ago? Absolutely," said Dean Stoyer, spokesman for Nike Golf. "He's certainly has been the cornerstone for our business, lending credibility and brand awareness, allowing us to create a golf company within Nike."

Woods has a reported $100 million, five-year contract with Nike that runs through 2006, with a variety of option years after that. But Nike's gotten a nice return on those endorsement dollars, as its golf sales reached nearly $500 million last year, according to Golf World Business.

So Nike and other blue-chip sponsors, like Buick, American Express and Disney, might not be worrying yet. (Buick just extended his endorsement deal another five years.) But even Stoyer admits that things go better for Nike Golf when Tiger wins the big ones.

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"On-course play is critical for product credibility and reputation," Stoyer said. "When he's winning and playing extremely well, it resonates and reflects at retail."

Stoyer points out that Nike Golf has endorsement deals with other pro golfers. "You can't build your business around one player," he said.

Even so, Stoyer concedes, "Nobody can move product like Tiger can."

When Woods isn't among the leaders, there aren't nearly as many people watching or following golf. His 2000 and 2002 U.S. Open wins drew 8.1 and 8.9 ratings for the Sunday play, respectively. In 2001 and 2003, when he wasn't on the leader board Sunday, ratings plunged to 3.7 and 4.9.

Endorsement experts says Woods' relative struggles have done little to damage the positive image that advertisers find almost as valuable as on-field or on-course success.

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The monthly survey of celebrities' public image by Marketing Evaluations, Inc., the research firm that does the famous "Q ratings" used by advertisers, found no change in this March's survey from the March 2003 ranking for Woods.

MEI found that 88 percent of sports fans knew who Woods was, and 39 percent considered him one of their favorites. That was higher than any other active athlete in any sport. No other active golfer is close in either measure. And Tiger had virtually no negative ratings.

"His positives outweigh negatives (in different groups surveyed) by multiples of two, three, four, even five to one," said Steve Levitt, the firm's president.

University of Delaware professor John Antil agrees that Woods' image is still untarnished. But he says that if the slump gets worse or continues for another couple of years, it must affect even Woods' endorsement potential.

"We have very high expectations for him. That's why when he's not winning a major, he's in a slump," said Antil. "If we go much longer without him winning a major, another year or two, it could be a problem. If he were to go into a true slump, not making cuts, not winning, having his rankings fall out of a top 10, I'd start worrying."

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One way that Woods could deal with a prolonged slump is to make commercials that make light of his difficulties, Antil suggested. It would be a sign of humility that can help build a maintain a positive impression of a celebrity endorser who is having problems.

A current Nike spot features David Duval, a former top-ranked player who has fallen on much harder times than Woods and hadn't entered a tournament for seven months before this weekend's open.

The commercial, which shows Duval and other golfers gathered around Woods' golf bag in a garage looking at a new Nike driver. When Duval takes a practice swing, he breaks the window of Woods' SUV in the process. The animated driver cover in the commercial yells, "Best contact you've made all year, Duval."

Woods hasn't gotten to the point where he needs to be poking that much fun at himself. Nike and his other sponsors better hope it doesn't get that far.