Serena Williams vs. Kiki Bertens: Score and Reaction from 2015 US Open

Kiki Bertens didn't make things easy for Serena Williams in their second-round matchup at the U.S. Open on Wednesday. But the No. 1 seed still found a way to win, knocking off her Dutch challenger, 7-6 (5), 6-3, and keeping her dreams of the calendar Grand Slam intact.

It was Williams' 30th straight win at a major, per ESPN Tennis on Twitter. 

"I've been really relaxed, though today I was a little tight," she said after the match, via the ESPN broadcast. "I think it showed. I hope I can get back to where I was before today." 

Williams struggled with her serve throughout, finishing with 10 double faults (though she also ripped seven aces). While she won 88 percent of her first-serve points, she struggled when forced to go to her second serve, winning only 10 of 28 such points (36 percent).

She also led Bertens in unforced errors, 34-31, though she managed to turn an afternoon full of shocking errors into another victory.

After splitting the first two games, Bertens pulled off a pretty big surprise, breaking Williams in the third game. But Williams would recover, breaking Bertens before winning the first set in an epic tiebreaker.

Williams struggled at times in the set, though her confidence remained.

"Being down a lot, I know I can try to make a comeback," Williams said. "I never stop, I always keep going and give it the best effort I can."

It was certainly an eventful final point, as Steve Tignor of Tennis.com noted:

It was also a shockingly close call for the No. 1 seed, as Richard Deitsch of Sports Illustrated tweeted:

For the player who has gone 50-2 this season, the biggest challenge in New York may not come from her opponent, as Jon Wertheim of Sports Illustrated noted:

The second set wasn't quite so dramatic, however.

Williams appeared to have momentum after winning the first-set tiebreaker, but Bertens didn't relent, and the pair split the first two games of the second set. Williams broke Bertens in the fifth game, giving her the opening she needed to put her opponent away before breaking her again in the ninth and final game.

Williams will next face American Bethanie Mattek-Sands, who knocked out Coco Vandeweghe on Wednesday, 6-2, 6-1, and she's already preparing, as Pam Shriver of ESPN shows:

"She has a very aggressive game," Williams said of Mattek-Sands after the match. "I wasn't surprised with her win today. She's a great player and a great person. I don't think it's going to be an easy match at all."

If Williams takes down Mattek-Sands, either No. 19 Madison Keys or No. 15 Agnieszka Radwanska will be waiting in the fourth round. Advance again and her sister, No. 23 Venus Williams, could be awaiting her in the quarterfinals.

But at this point, the biggest challenge for Williams will be remaining focused, ignoring the pressure of making history and simply playing her game.

She remains the prohibitive favorite to win this tournament, even after a shaky second round, and it would be more surprising if she didn't win the title in New York than it will be if she does.