By Steve Gorten

KEY BISCAYNE – It’s always “tricky” for Agnieszka Radwanska when she faces an opponent for the first time.

That was the situation for her second-round match of the 2015 Miami Open on Thursday, and predictably, the world’s No. 8 ranked player struggled at times against No. 58 Anna Schmiedlova.

But the 2012 tournament champ and No. 7 seed this year still managed to advance in straight sets with a 6-4, 7-5 win.

“I was in trouble, especially in that second set,” said Radwanska, who trained in Miami for five days with new co-coach Martina Navratilova earlier this year. “What was most important is that I was really playing good shots in the really key points, in the break points, and I used that well.”

Unfortunately for Radwanska, who has reached the quarterfinals of the Miami Open the past five years, there will be no chance to play her younger sister, Urszula, in this year’s quarters should she advance.

The possibility of that matchup ended hours after Agnieszka’s win when No. 16 seed Venus Williams defeated Urszula 6-3, 6-2 on stadium court.

A set of sisters going head-to-head might still happen: Venus and her younger sister Serena, who will begin her run for an eighth Miami Open title Friday night, could potentially meet in the final.

Venus Williams’ triumph Thursday capped a day session that set a tournament record for attendance (14,827) that topped the previous record from 2011 by 336 fans. Thursday’s sellout was the second of this year’s Miami Open.

A huge portion of Thursday’s crowd was there to see, and support, Argentina’s Juan Martin del Potro in his first-round match against Canadian Vasek Pospisil. The former No. 4 player in the world, who’s now No. 616, was playing his first match since undergoing a second surgery on his left wrist following three matches at Sydney in January. Del Potro played just four tournaments last year, missing the final nine months after his first surgery.

Hampered by a two-handed backhand Thursday that wasn’t close to his top form, del Potro fell to Pospisil 6-4, 7-6 (9) in front of a stadium court crowd strongly rooting for him.

“Juan Martin hasn’t played many matches, but it’s still tough going out there because he’s still a big name,” Pospisil said. “That along with the [windy] conditions made it tough. But given the circumstances, I thought I played a great match.

“I didn’t really know what to expect [from del Potro] and that made it a little bit more difficult,” he added. “I wanted to play my game, try to put pressure and come to the net. I executed that game plan pretty well.”

Pospisil not only didn’t mind the crowd rooting against him, he said it spurred him.

“To be honest, I was enjoying it. It was like a Davis Cup atmosphere. I felt like I was playing away,” Pospisil said with a smile. “It was a great atmosphere, whether everyone was cheering against me or for me or it was equal. When there’s a big crowd and they’re into it, it’s more enjoyable for everybody.”

Pospisil said it was obvious to him that del Potro’s backhand wasn’t the same as usual.

“I tried to take advantage of that by coming in and putting pressure on that side,” he said. “He’s still serving well, and his forehand is huge, which made it tough. I just hung in there, got maybe a little fortunate at the end.”

Del Potro led the tiebreaker 5-2, but double-faulted on set point. Then after serving an ace on match point for Pospisil, he double-faulted again. After an errant forehand from del Potro the next point, it was over.

“I don’t feel frustrated,” del Potro said. “I have to take the positive things on my comeback. Just playing another [major ATP] match after one year, it’s a good signal for the future. …It’s only one match, but it was enough for the beginning.”

Del Potro said he fell into depression in the past year due to his injury, and that moving forward, “it’s most important to be patient and stay calm and try to be positive.”

In other afternoon action Thursday, No. 4 seed Caroline Wozniacki shook off a third-round loss at Indian Wells with a 6-0, 6-1 rout of American Madison Brengle that took just 57 minutes. Eighth-seeded Ekaterina Makarova beat Karin Knapp 6-1, 6-3.

Other women’s winners included Andrea Petkovic, Kaia Kanepi, Irina-Camilia Begu, Alize Cornet, Carla Suarez Navarro, Elina Svitolina, Kristina Mladenovic, Andrea Petkovic, Karolina Pliskova and Paul Badosa Gibert.

On the men’s side, Martin Klizan, Alexandr Dolgopolov, Mikhail Kukushkin, Alejandro Falla, Alexander Zverev, Mikhail Youzhny, Simone Bolelli, Borna Coric, Jerzy Janowicz and Teymuraz Gabashvili advanced.

Thursday also marked the start of men’s and women’s doubles.