If you are playing a tough point that involves running from baseline to baseline as fast as you can, you are likely using anaerobic mechanisms during that point.
But once the point stops, and for the next 15 seconds before the next serve you are likely using aerobic calories. And your heart rate is starting to fall.
And a game may be over in as little as 4 points.
Aces or missed returns result in our heart rate falling.
Many points are over after just a couple of hit balls.
We change sides after every other game.
Few play best of 5 set matches.
Most play tiebreakers.
Although tennis is a "good workout", there is plenty of time for the heart rate to fall so that a heart rate monitor is probably not the best indicator of whether a tennis player is using aerobic or anaerobic mechanisms.
I certainly can "feel" more taxed playing full court pickup basketball (without refs for frequent whistles), than in playing tennis.
I haven't worn my heart rate monitor playing basketball. (One team is usually "skins" - it would just look too weird.)
But wearing it a few times playing tennis the heart rate stays much higher with continuous rally practice using 6 balls than playing sets.
Still, I think I am burning calories anaerobically during the long points playing sets.