By Steve Gorten

KEY BISCAYNE – Monica Niculescu can frustrate opponents with her patented forehand slice — the tennis equivalent of a knuckleball, and the shot she hits regularly in lieu of the topspin forehand that’s typical on the WTA tour.

Even the world’s No. 1 player, Serena Williams, struggled some against Niculescu’s unique style when she faced her for the first time at Indian Wells earlier this month.

But the second time around was no issue for Williams.

The seven-time Miami Open champion started her 2015 campaign with a 6-3, 6-1 win against Niculescu on stadium court Saturday.

“It was definitely easier today. I knew what to expect,” Williams said of handling Niculescu’s heavy spin. “I still got a little frustrated, but overall was able to stay calm more than the last time I played her.”

Williams won in straight sets as well at Indian Wells, but lost five games each set. As important as her performance Saturday was, the fact her injured knee wasn’t an impact on her game was much more important.

“I really didn’t feel it today, to be honest with you,” she said. “Once you step out on the court, you have so much adrenaline going. The adrenaline kind of kills it. So I was surprised. I felt pretty good.”

Williams withdrew before the semifinals at Indian Wells because of the injury, and described her knee as sore in practices at Crandon Park Tennis Center this week. She’ll have less rest than anticipated before her third-round match against American wild-card Catherin Bellis on Sunday because Williams and Niculescu were slated to play Friday night. Their match was one of a dozen matches pushed back to Saturday because of heavy rain.

It’s why Williams chose rest over going to grandstand to watch older sister, Venus, play Samantha Stosur following her win against Niculescu.

Venus Williams, the tournament’s No. 16 seed, edged the Australian Stosur 6-4, 7-6 (3) in a tightly contested match.

The day session Saturday drew a sellout crowd of 17,187 – the third session sellout of the tournament this year and the highest-attended session since Session 9 in 2012 (18,656).

The surprise of the afternoon was qualifier Tatjana Maria’s 6-0, 7-6 (4) upset of sixth-seeded Eugenie Bouchard on Court 2. By the end of the match, the bleachers were packed with spectators, and a long line of people waiting to get in snaked outside.

The 21-year-old Canadian Bouchard, ranked seventh in the world, was on course to play No. 3 Simona Halep in the quarterfinals.

Unlike Bouchard, fourth-seeded Caroline Wozniacki avoided an upset loss. After losing the first set to Kaia Kanepi, Wozniacki rallied for a 4-6, 6-1, 6-3 triumph. That set up a fourth-round match with Venus Williams.

Eighth-seeded Ekaterina Makarova, Carla Suarez Navarro and Daria Gavrilova, the wild-card who beat Maria Sharapova on Thursday night, also advanced Saturday.

On the men’s side, fourth-seeded Kei Nishikori reached the third round with a 6-2, 6-1 win against Mikhail Youzhny. Americans John Isner and Jack Sock also advanced in straight sets while countryman Tim Smyczek fell 4-6, 6-3, 3-6 to 11th-seeded Jo-Wilfried Tsonga.

Ninth-seeded Grigor Dimitrov dispatched Canadian Vasek Pospisil 6-2, 6-2. Pospisil had advanced with a win against fan favorite Juan Martin del Potro.

Other winners in the men’s draw included 12th-seeded Giles Simon, David Ferrer, Gael Monfils, Jeremy Chardy, Lukas Rosol, Fernando Verdasco, Alejandro Falla, Steve Darcis, Juan Monaco, Adrian Mannarino, Viktor Troicki, Jerzy Janowicz and Alexandr Dolgopolov.