The problem is "nerves" don't come out and tell you what is wrong, and mostly you just feel uncomfortable, want to start the match again, feel tentative, but, what are the real signs:

• uncoordinated
• choking the swing
• hitting and standing
• feel heavy on your feet
• want easy points from the Ref
• jerking body parts
• walking between points quickly

Compose yourself before going on court (assuming you have already warmed-up and stretched):

• take some time to focus on the body feelings
• relax the muscles
• swing the racket to get rhythm and timing
• visualise perfect shots, eg drives, serves
• make no eye contact with your opponent or audience
• increase the heart rate before stepping on court, eg skip to elevate the heart.
  

The hit up:

• try not to win the hit up
• hum a tune to help improve the rhythm
• breath slowly and deeply
• take your time to get a good feeling in the strokes

The first game:

• Settle down by making no errors
• Go for the shots but not too fine (close to tin)
• Strive to implement the ten fundamentals, eg firstly good length, tight length, return the serve on the volley if possible, get to the T.
  
• Attack when in position; tight defence when out of position.
• Controlled breathing, after a long rally, will improve oxygen to the brain, and make you think clearer.
• Play the game your way.
  

If all this fails:

• relax and enjoy the game; sometimes that can settle things down too.