Industry tracker SearchIgnite said on Tuesday that Google has tightened its powerful grip on US internet advertising revenue but a Bing-Yahoo! alliance is fielding a viable challenge.Google's share of spending on pay-per-click (PPC) advertising at online search services grew to 80 percent in the third quarter of this year while Bing claimed 6.4 percent and Yahoo!'s portion slipped to 13.4 percent.Microsoft search engine Bing and Yahoo! have been testing making money from search advertising as part of an alliance arranged to join forces against market king Google.Google chief executive Eric Schmidt last month said that Microsoft's Bing search engine was the Mountain View, California-based company's main threat."Absolutely, our competitor is Bing," Schmidt said in a Wall Street Journal interview video posted online. "Bing is a well-run, highly competitive search engine."Yahoo! and Microsoft forged a Web search and advertising partnership a year ago that set the stage for a joint offensive against Google.Under the agreement, Yahoo! will use Microsoft's search engine on its own sites while providing the exclusive global sales force for premium advertisers.Microsoft has begun handling all Yahoo! online searches in Canada and the United States and will eventually power Internet searches at Yahoo! websites worldwide.