Grand Slam champions Sofia Kenin, Jelena Ostapenko and Sloane Stephens all booked spots in the second round of the WTA 500 Cymbiotika San Diego Open on Tuesday.

2020 Australian Open champion Kenin pulled off a speedy 6-4, 6-4 upset of No.6 seed Veronika Kudermetova to kick off the day.

Former World No.4 Kenin, currently ranked No.93, garnered the 1-hour and 25-minute win to improve to 2-1 against 18th-ranked Kudermetova. The victory marks Kenin's 20th Top 20 win of her career.

Kenin used a fierce forehand to force an error from Kudermetova and earn the only break of the opening set, moving ahead 4-3. Three games later, the American cracked a backhand winner down the line to convert her second set point.

Kudermetova held the early lead in the second set, as she moved ahead by a break at 4-2. But Kenin fired a forehand crosscourt winner to immediately pull back on serve, kicking off a run of four straight games to take the win.

Wild card Kenin will face another American wild card in the second round: Katie Volynets, who defeated American teen Clervie Ngounoue in the first round on Monday.

Later on Tuesday, 2017 Roland Garros champion Ostapenko came back from 5-2 down in the third set to outlast No.8 seed Ekaterina Alexandrova 7-6(3), 6-7(6), 7-5 in 2 hours and 47 minutes. It is only Ostapenko's third win over Alexandrova in their eight career meetings.

World No.16 Ostapenko, who knocked out defending champion Iga Swiatek at last week's US Open, held a match point at 5-3 in the second set, but 19th-ranked Alexandrova came all the way back in the set, eking it out in a tiebreak and leveling the match.

Alexandrova then built a commanding 5-2 lead in the third set and served for the match, but Ostapenko cracked a return winner to convert her fourth break point of that game and extend the affair.

That started another purple patch for Ostapenko, who eventually won 10 points in a row en route to a 6-5 lead. In the following game, Ostapenko slammed another forehand return winner to clinch her fifth straight game and close out the match. 

Ostapenko will now take on another powerful player in the second round: 2022 Australian Open finalist Danielle Collins, who picked up a first-round win over Louisa Chirico on Tuesday. Ostapenko leads their head-to-head 2-0, including a win this year in Abu Dhabi.

The third and final Grand Slam champion to play on Tuesday, 2017 US Open titlist Stephens cruised past Elise Mertens 6-1, 6-3 in just 1 hour and 13 minutes.

World No.38 Stephens was stellar on return in the evening first-round clash, winning 69 percent of 29th-ranked Mertens's second-serve points and converting six of eight break chances.

Stephens will now take on the second seed in the second round: World No.10 Caroline Garcia, who had a first-round bye. Stephens and Garcia have faced off seven times, with Garcia holding a slim 4-3 edge in their rivalry.