Top favorites and 2016 Grand Slam champions Angelique Kerber and Garbine Muguruza both advanced into the second round of the US Open on Monday.

Kerber, who won the last hardcourt Slam in Australia, won the first seven games of her match against Polona Hercog before Slovenian retired because she wasn’t feeling well in the heat.

“It’s never the way I would like to finish the match, but I went out there to feel my rhythm and start the tournament well,” Kerber said. “I played the first set really good, so this is what I will take from this match, that I’m playing my tennis. For me, it’s always tricky the first few rounds. So it’s always good, yeah, to have the first round done. Just now focusing on the next rounds.”

Of course Kerber is once against trying to reach the No. 1 ranking, and she’ll have it if Serena doesn’t reach the semifinals.

“For sure to be one day No. 1, I think this is a goal from everybody, especially also for me,” she said. “But I will not putting too much pressure on myself like I said a lot of times, because I know that when I put the pressure I’m not playing my tennis then.

“I will go out there to win every match going step by step. If the day will come someday it will be amazing. But, yeah, just let’s see. I have to win few more matches.”

While Kerber had an easy day, Muguruza was down a set to qualifier Elise Mertens before getting back on track to win 2-6, 6-0, 6-3.

“I started and it was difficult a little bit to breathe for me,” Muguruza said. “It was really humid. I don’t know. When you feel the heat that makes you a little bit, like, down kind of, you know. I was talking in the locker room a little bit with the physio. Was kind of like similar to Australia, you know, where is hot.”

Muguruza received treatment and played better. And she also lost the first set in Paris before going on a run all the way to the title.

Former No. 1 and 2-time finalist Caroline Wozniacki, unseeded at No. 74, was down a set also to qualifier Taylor Townsend before she won in three sets.

Also, Petra Kvitova, Dominika Cibulkova, Belinda Bencic and former champion Svetlana Kuznetsova were also winners.

Americans CiCi Bellis, Shelby Rogers, Christina McHale and 16-year-old Kayla Day advanced. Rio gold medalist Monica Puig fell to Saisai Zheng.

In the late night match (really late!), Madison Keys and Alison Riske were locked in a tight battle.

On Tuesday, the Williams sisters Serena and Venus return along with Simona Halep and Agnieszka Radwanska.