MELBOURNE: Shelby Rogers thoroughly dominated the No. 4 player in the world, Simona Halep. A teary-eyed CoCo Vandeweghe battled nausea to outlast another top-20 veteran, Roberta Vinci.

And the grande dame of the women's tour, 36-year-old Venus Williams, refused to cede the spotlight for at least another match at the Australian Open, beating Kateryna Kozlova in the first round of her 73rd career Grand Slam tournament, an Open-era record.

It's been a fast start for many of the 32 Americans in the men's and women's singles draw in Melbourne — both the old guard and the unproven youngsters of the next generation. Thirteen won their first-round matches on Monday, compared to six losses — two of which were to American opponents.

The power-hitting Rogers scored the biggest upset of the day. Newly confident after her surprising run to the French Open quarterfinals last year, the 24-year-old South Carolina native outplayed Halep with an aggressive, all-court game, winning 6-3, 6-1.

"The biggest thing I took away from (the French Open) was just that I can compete with the top players in the world," she said. "(There are) little things here and there I need to work on, but I'm here. I need to believe in myself."

Vandeweghe fell to her knees with nausea while trailing in the second set of her match against Vinci, but after an extended medical timeout and a couple sips of a soft drink, she recovered to break back and finish off a 6-1, 7-6 (3) victory.

Her first thought, she said, was "I'm going to be that person who's going to barf on the court. And I hate throwing up."

After a couple of minutes on her knees, she felt well enough to continue. The sugary drink helped, too.

"Kind of just out of habit with a grandfather that's a doctor. He always said the syrup in Coke coats the stomach."

The 13th-seeded Williams had dramas of her own against Kozlova, rallying twice from a break down in the opening set before prevailing 7-6 (5), 7-5. When she finally closed out the first set with an ace on her third set point, the normally stoic Williams bent low, pumped her fist and let out a roar.

Venus made clear after the match — in case it wasn't obvious — that she still has a passion for the game, 19 years after her Australian Open debut.

"I'm not coming all the way to Australia for kicks and giggles. I'm here as a competitor," she said.

On the men's side, the 19th-seeded John Isner, 23rd-seeded Jack Sock and 31st-seeded Sam Querrey notched first-round wins, as did Ryan Harrison, a one-time rising star who has rediscovering his love for the game after years of unhappily trying to live up to unmet expectations for his career.

With a number of young Americans in the draw at Melbourne Park, including teenagers Taylor Fritz, Michael Mmoh and Frances Tiafoe, Harrison feels like he can serve as an example.

"I've been through the ups of knowing how fun it is when everyone's behind you and talking about you can win Grand Slams, and I've been through the downs of everyone saying that you're literally done with your career," said the 24-year-old Harrison, who beat Nicolas Mahut 6-3, 6-4, 6-2.

"You can get so caught up in this obsession with results that you forget to enjoy the aspect of how fun it is to play at this level."

Serena Williams headlines the Americans playing on Day 2 as she kicks off her campaign for a 23rd Grand Slam title against Belinda Bencic.