Rivals to advertising groups Omnicom and Publicis are looking to grab business from clients dissatisfied with the merger of the firms.
The rival companies include market leader WPP, Interpublic Group, which is the fourth-biggest, and Japan's Dentsu. Omnicom and Publicis are the second and third-largest advertising companies, respectively.
The three companies said they had either started winning business from clients of Omnicom and Publicis or believe to do so in the near future. They are also looking to grab more than 130,000 Omnicom and Publicis staff across five continents who could move out from the merged group.
"It certainly has disturbed the client base and it certainly has disturbed the staff," WPP CEO Martin Sorrell told an investor conference in Barcelona.
"Clients are not going to come out and say 'I'm firing an agency' because they merged. But if you watch the rooms carefully, there are changing patterns of distribution in the business which will benefit us."
Interpublic CEO Michael Roth said his company had recently hired a number of "talented people" from the merging firms.
"When you have two companies of that size coming together, with different cultures, there's going to be disruption. And from disruption comes talent opportunity," Roth said.
"There are still some conflicts out there that are unacceptable and we are a very viable candidate if that is an issue. We are ready, willing and able to step in."
WPP and Interpublic reported organic growth of 5% and 2.8%, respectively, in the third quarter, compared to 4.1% and 3.5% growth rates achieved by Omnicom and Publicis.
Meanwhile, Japan's Dentsu that completed its acquisition of British media buyer and planner Aegis earlier in 2013 said it has also started benefitting from the development.
"We've had some success when you look at our new business record in the past quarter, we've had a high percentage of that business coming from Publicis Omnicom," Dentsu chief Tim Andree said.
'No Threat to Business'
Omnicom and Publicis, however, do not expect major business losses from the merger, despite the pressure from rivals.
Omnicom CEO John Wren and Publicis boss Maurice Levy said they continue to believe in the combined group's ability to compete in the changing media landscape. They noted that the support of their largest clients remains.
"I don't see any great threats coming from client conflicts," Wren said.
"People have tried to promote that as an issue but that's not real in my opinion."
Publicis and Omnicom announced their merger in July, creating the world's largest advertising group valued at about $35bn (£21.7bn, €26bn). The merger is due to be completed in the first half of 2014.