From Friend at Court:
30. Delays during service.
When the server’s second service motion is interrupted by a ball coming onto the court, the server is entitled to two serves.
When there is a delay between the first and second serves:
• The server gets one serve if the server was the cause of the delay;
• The server gets two serves if the delay was caused by the receiver or if there was outside interference.
The time it takes to clear a ball that comes onto the court between the first and second serves is not considered sufficient time to warrant the server receiving two serves unless this time is so prolonged as to constitute an interruption. The receiver is the judge of whether the delay is sufficiently prolonged to justify giving the server two serves.