Roger Federer has revealed the knee surgery he underwent last month was required due to an injury sustained while preparing a bath for his twin daughters.

The 34-year-old plays Juan Martin del Potro on Friday in the second round of the Miami Open - the Swiss' first tournament appearance since injuring his left knee following his Australian Open defeat to Novak Djokovic in January.

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Federer had to undergo arthroscopic surgery to repair a torn meniscus, and clarified the cause of the problem on the eve of his return.

"It happened the day after the Djokovic match," the 17-time grand slam champion said.