I read your sentence and parsed it correctly at first take. It is certainly a correct sentence.
That said, not only is moustic is right about the potential ambiguity in 'as', there is also a (largely theoretical) potential for confusion in that the subject changes from 'him' in the first clause to 'his brother' in the second. Consider these two possible contexts:
1. "While he was playing tennis, his brother was watching him. Again. His brother always watched, calling out unwanted advice from the sidelines and putting him off his stroke."
--> Here it is (I hope) clear that 'he' is playing, 'his brother' is watching.
2. "Sam reached into his pocket for his secret hip flask. At last, with his brother focused on playing, he could sneak a drink. Then he froze. While he was playing tennis, his brother was watching him! It was outrageous! Would nothing distract him for long enough for Sam to take even a sip? Again he cursed the day his brother had Found God and decided to turn Sam teetotal."
--> Now it is the brother playing, not 'him.'
In reality the misunderstanding is unlikely, and my contexts are pretty forced; but it is a semantic possibility. **YOU DON'T NEED TO WORRY ABOUT IT!!!** I'm a bit of a language nerd, that's all, and such quirks interest me.