"Once the toss went up and knowing that he was going to play, I certainly understood what he was going to bowl to me," Watson said of Kerrigan, who had 0-53 from eight overs.
"After the last tour match I certainly had a game plan of what I was going to do and I sort of followed on from that in this innings."
With the series already wrapped up, England opted for two spinners at home for the first time since 2009 and also handed a first cap to fast bowler Chris Woakes (0-52).
Watson had expected to be facing hulking paceman Chris Tremlett, who is yet to get a look-in this series.
But he wasn't complaining.
"I was certainly expecting him [Tremlett] to play, more so than Woakes or Kerrigan," Watson said.
"Certainly I'm a bit happier not to face a guy who is six foot eight bowling balls that are bouncing up into your splice all the time."
Paceman James Anderson (2-52) took the crucial wickets of David Warner and Michael Clarke to move ahead of former fast bowler Bob Willis into second on England's all-time list of test wicket-takers with 326.
- AAP