KEY BISCAYNE, Fla. -- Roger Federer wasn't playing tennis when he hurt his knee two months ago. He was playing dad.

Federer said he was preparing a bath for his twin daughters when he turned and heard a click in his left knee, leading to the first surgery of his career Feb. 3.

"It was a very simple movement, probably a movement I've done a million times in my life," Federer said Thursday. "I didn't think much of it when it did happen."

Soon his knee was swollen and required arthroscopic surgery to repair torn cartilage. He's scheduled to return from his layoff Friday against longtime rival Juan Martin del Potro at the Miami Open.

Federer, 34, has been a model of durability throughout his career. He's playing Key Biscayne for the 16th time, although he did skip the tournament last year for scheduling reasons.

Federer made a late decision to enter the event this month, surprised and pleased by his speedy recovery. He was on crutches for 12 days and has trained without restrictions for the past nine days.

"Expectations are really low, which is nice for a change -- just see where I am," he said. "I'm just really pleased I'm back. I didn't expect myself to be back here, to be quite honest, after the surgery."

Winners on the women's side included No. 1-seeded Serena Williams, who began her bid for a ninth Key Biscayne title by beating fellow American Christina McHale 6-3, 5-7, 6-2. Williams earned her 19th consecutive victory in the tournament and hasn't lost since the 2012 quarterfinals.

Facing McHale for the first time, Williams converted only five of 17 break-point chances. She let a match point get away in the second set and had to play for an additional 50 minutes, losing five consecutive games before she regrouped.

"Christina just started playing better, and playing tennis she has never played before," Williams said. "I made a few mistakes and then I just had to pull myself back together."

Wild-card Heather Watson beat 20th-seeded Sloane Stephens 6-3, 6-0. No. 3 Agnieszka Radwanska and No. 7 Petra Kvitova also advanced.

Roger Federer is scheduled to return to action Friday, when he faces Juan Martin del Potro in the Miami Open. Quinn Rooney/Getty Images

Federer, who is seeded third, had a first-round bye. His recent layoff came during an eventful stretch for his sport, with Maria Sharapova's career in jeopardy following a failed a doping test, and renewed debate about equal prize money for men and women.

Federer offered his thoughts on each subject and said he was "completely surprised" by Sharapova's suspension. He said he doesn't believe tennis has a doping problem but would like to see more consistency in testing.

"I've been in Dubai now for 10 years there and been tested once," he said. "That's not OK for me. I get tested more in Switzerland because the guy from Switzerland lives in my village. He comes and sees me the day after my surgery, and one week later."

As for equal prize money, Federer said he's all for it.

"I'm happy that tennis has produced some of the greatest female athletes in the world," he said. "Equal prize money is a good thing."

The first marquee match at Key Biscayne will be on the men's side, with Federer facing another Grand Slam champion mounting a comeback from an injury. Del Potro returned to the tour in February after an 11-month layoff during which he twice had surgery on his left wrist.

"His injury was much, much greater," Federer said. "That's why I'm really pleased for him that he was able to find a way back onto the tour."

Federer is 15-5 against del Potro, including 5-1 in Grand Slam tournaments. That one loss came in the 2009 US Open final for del Potro's lone major title.