WASHINGTON, DC, USA - Teenagers Aryna Sabalenka and Bianca Andreescu sent shockwaves through the opening day at the Citi Open on Monday as each posted upset victories. 

Sabalenka stunned last year's finalist and No.3 seed Lauren Davis, 7-5, 6-3, while Andreescu became the youngest Canadian in nearly 30 years to win a WTA main-draw match with a 5-7, 6-3, 6-4 win over Camila Giorgi.

Andreescu is the youngest from her country to win a main-draw match since Maureen Drake in February of 1988.

"It honestly feels amazing, this is my first WTA win so I'm just super happy I pulled it out in the end," she said. "She was such a good player, it was hard to find a rhythm with her. I just tried to stay focused and put pressure on her, especially her forehand - and I just really focused on my serve, because her serve was really good and I had trouble returning sometimes. I think I really wanted it."

Andreescu awaits the winner of the match between the No.2 seed Kristina Mladenovic and Tatjana Maria. The 17-year-old was full of praise for the Frenchwoman on Monday: "Mladenovic is a really good player, I really admire her, so I think it's going to be a good match."

"I'm so happy - during the match, I was really focused on my game and trying to play my best," Sabalenka said. "It's my first WTA [tournament] win - I feel like I can do everything on the court, so I will try and keep this feeling throughout the tournament. I just tried to keep aggressive in the match, so maybe that's why I won."

The Belarusian has been in form in recent weeks, as she qualified for Wimbledon to make her Grand Slam main draw debut and reached the second round. The World No.121 rallied from a break down twice in the opening set, saved a set point as Davis served for the opener at 5-4, and won the last three games of the set.

In the second set, Sabalenka again trailed by an early break, but she won four straight games to put herself in command of the match, and saved a break point in serving out the match.

Action around the grounds on Monday also saw No.4 seed Julia Goerges advance to the second round with a 6-4, 6-4 win over Andreescu's compatriot Francoise Abanda, while Sara Errani ousted Risa Ozaki, 7-6(0), 6-2. No.5 seed Oceane Dodin advanced leading 4-6, 6-3, 2-0 when Jelena Jankovic retired in the decider.

Also advancing were Andrea Petkovic and Alison van Utyvanck, who won their opening matches in contrasting fashion. Petkovic edged Kurumi Nara in a tight two-setter, 6-4, 7-6(5), while Van Utyvanck came out on top in a roller-coaster match against Jana Cepelova, 7-5, 0-6, 6-1.