Katerina Siniakova of the Czech Republic pulled off a second-round upset at the Qatar TotalEnergies Open on Tuesday, ousting No.2 seed and reigning US Open champion Coco Gauff 6-2, 6-4.

Ranked No.42, Siniakova clawed back from 4-0 down in the second set before grabbing a 1-hour and 20-minute victory over Gauff. The Czech collected just her second win in seven meetings with the 19-year-old American.

Career-best Doha result: Siniakova, a former WTA Doubles World No.1 who has won seven Grand Slam doubles titles, moves into the Doha Round of 16 in singles for the very first time in her career.

Siniakova will face another accomplished American, Danielle Collins, in the Round of 16. Former World No.7 Collins, the 2022 Australian Open finalist, qualified for the main draw this week and has already beaten No.13 seed Veronika Kudermetova and Marie Bouzkova.

Siniakova and Collins have split their two previous meetings. Siniakova beat Collins from a set down at 2019 Toronto, but Collins recently avenged that loss in last year's Billie Jean King Cup Finals.

"Definitely another tough match, [Collins is] a really aggressive player," Siniakova said, looking ahead to her next round. "I will try to focus on my game and fight every point, and hopefully it will be a really good game."

Must See

Stat corner: With her victory over World No.3 Gauff, 27-year-old Siniakova notched the fourth Top 5 win of her career. It was the Czech's first win over a player ranked inside that echelon since she ousted then-World No.1 Naomi Osaka in the third round of 2019 Roland Garros.

Expanding out further, Siniakova earned the ninth Top 10 win of her career on Tuesday, after going 0-4 against Top 10 players last year. Six of Siniakova's Top 10 wins have come on hard court.

Siniakova won 70 percent of points returning Gauff’s second serve during Tuesday's clash. The unseeded player went 6-for-14 on break points, while Gauff was 3-for-8.

Match moments: Siniakova broke Gauff in the opening game of the match on her way to a 4-1, double-break lead. Serving for the set at 5-2, Siniakova garnered triple set point with her first ace of the day, then closed out the love hold after a wide return by Gauff.

Gauff’s championship form held sway in the early stages of the second set, as she fired a passing winner to reach 4-0. However, Siniakova methodically pulled her way back into the set, saving a break point at 4-1 and pulling back on serve with a love break for 4-3.

After a long forehand misfire by Gauff, Siniakova grabbed a third straight break of serve to lead 5-4 and serve for the match. At 30-30, the Czech hit a sturdy backhand to line up match point, which she converted to wrap up the victory.