Longtime rivals Agassi and Sampras were wearing microphones during the match, and the crowd at Indian Wells Tennis Garden in California could hear an exchange that started lighthearted but turned testy.
At one point Sampras mimicked Agassi's pigeon-toed walk, which drew laughs from the crowd.
Then Agassi chided Sampras for being stingy. In his book, Agassi criticized Sampras for being a poor tipper.
Sampras responded with a high, hard serve that forced Agassi, who was partnering Rafael Nadal, to duck.
Both Agassi and Sampras were smiling, but neither showed up at the post-match news conference.
"It was out of line," Agassi told ESPN.com on Monday. "It was inappropriate. The night was on fire. We were all having fun. I was trying to be comedic. I only had a split second to make a decision. I went for it and it fell flat. I was trying to get past it, but Pete didn't really let me get past it. He didn't really roll with it. I've texted Pete to ask him if I can apologize in person."
"The joke fell flat and I'm sorry," he added. "My hope was that the night was still enjoyable."
Nadal was asked about Sampras and Agassi after his Monday night match.
"Everybody told me after the match what's happened, but during the match, you know how fast Americans speak, and I am Spanish. I didn't understand nothing," he said.
He said he felt no tension between them either on or off the court.
A capacity crowd of 16,100 filled the stadium for the Hit for Haiti fundraising exhibition Friday night.
The event helped raise $1 million to aid relief efforts in the earthquake-ravaged island nation.