Henman said: "My record is a good one but he has come a long way since we played early on. I'm going to have to play with authority."
Andre Agassi moved one match closer to winning his sixth Legg Mason Classic title with a 6-3, 6-2 victory over Sargis Sargsian of Armenia.
Top seed Agassi has won 40 matches and only lost nine in his 15 tournament appearances in Washington.
And the American needed just one good return game in each set to beat the 13th seed Sargsian for the sixth straight time and advance to the quarter-finals.