A bevy of stars contested their first-round matches in Tuesday's afternoon session at the Citi Open, including three Grand Slam singles champions. Here is how the daytime matches concluded...

Ajla Tomljanovic serving at the Citi Open on Tuesday.

Richard Kessler/Citi Open

Ajla Tomljanovic def. Sloane Stephens 6-1, 6-4: Tomljanovic came into the match with an 0-6 record against Stephens, but the Australian prevailed in 68 minutes to collect her first victory over the 2017 US Open champion.

"It got a little tricky at the end," Tomljanovic said. "I definitely thought about the fact that I haven't beaten her so far and been close a few times, so I got nervous and played a few loose games, but just very pleased to close it out in the end."

Tomljanovic, who reached her second consecutive Wimbledon quarterfinal last month, converted five of her eight break points to notch the win. Stephens, who won her first WTA singles title at the 2015 Citi Open, was undone by 25 unforced errors.

Victoria Azarenka in action at the Citi Open on Tuesday.

Richard Kessler/Citi Open

(4) Victoria Azarenka def. Dayana Yastremska 6-4, 6-0: Making her Citi Open debut, former World No.1 Azarenka swept past Yastremska in 1 hour and 27 minutes to reach the second round.

Azarenka, who won back-to-back Australian Open titles in 2012 and 2013, claimed 60 percent of points returning serve to level her head-to-head against Yastremska at 1-1. 

Camila Osorio collected her first win since Roland Garros at the Citi Open on Tuesday.

Paul Staples/Citi Open

Camila Osorio def. Sofia Kenin 7-6(2), 6-1: Osorio topped 2020 Australian Open champion Kenin, ending the Colombian's five-match losing streak. Prior to this victory, Osorio's most recent win came over Harmony Tan in the first round of Roland Garros.

Coming back from injury, Kenin contested her first singles match since Indian Wells in March. Kenin fought back from 5-2 down to push the opening set into a tiebreak, but Osorio regrouped to win the breaker and eased through the second set from there.

In other day matches, former Top 10 player Andrea Petkovic dispatched 8th-seeded teenager Clara Tauson 6-2, 6-2 in 69 minutes to reach the second round. Petkovic, a Citi Open finalist in 2013, was a perfect 6-for-6 on break points.

Also, Anna Kalinskaya, a semifinalist at the most recent Citi Open in 2019, cruised past Madison Brengle 6-3, 6-0 in 71 minutes. And lucky loser Wang Xiyu ousted recent Wimbledon semifinalist Tatjana Maria 6-2, 7-5.