How often do they change the balls in tennis?

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Tags

1. #player
2. #every
3. #throughout
4. #usually
5. #accuracy
6. #first
7. #three
8. #happen
9. #starts
10. #alternately
11. #seven
12. #after
13. #around
14. #match
15. #balls
16. #games
17. #hours
18. #control
19. #changed
20. #affect