PARIS, France -- Another top seed fell by the wayside on Sunday in the first round of the French Open, as Kateryna Kozlova of Ukraine stunned defending champion and No.5 seed Jelena Ostapenko of Latvia, 7-5, 6-3.

"I didn't expect anything from the match," Kozlova said in her post-match press conference. "I just went to enjoy every moment on the court because, before, I was not sure if I will be able to play in French Open. So I was just happy to be on court, back on court, and compete. So it was very important for me. And, in the end, the result comes up and it's just amazing."

"I got injured in Indian Wells and I get hurt in the knee," Kozlova elaborated. "There was a chance, maybe, that I would have to go for surgery. But luckily I met very good physios and professionals, and I went for rehab to Tenerife, and they did a great job. And after two months, I come back, and I already played last week. So we avoid the surgery. That was the most important thing."

Another upset: Wang stuns Venus in French Open first-round surprise

The Ukrainian picked up a win over a player ranked in the Top 30 for the first time with Sunday’s victory, claiming her career-best victory in one hour and 35 minutes. Kozlova took two-thirds of points on Ostapenko's second serve, while the titleholder did herself no favors with 48 unforced errors, including 13 double faults.

Notably, Kozlova had previous success against the World No.5 with a 2-0 head-to-head record, and despite Ostapenko’s recent emergence as a Top 5 player, Kozlova continued to have the Latvian’s number, sending the 2017 Roland Garros champion out of the tournament on the first day of main draw play.

"I know that if I will play her game, that's what she wants, and I'm trying to play differently, and probably this is the reason [for the head-to-head]," said Kozlova. "But this match is different, of course. She had so much pressure."

Ostapenko is the first French Open defending champion to lose in the first round since 2004 winner Anastasia Myskina dropped her opening match to María Sánchez Lorenzo in 2005.

"I think it was terrible day at the office today for me," Ostapenko said to the media, after the match. "I mean, in general I played maybe like 20 percent of what I can play. Made like 50 unforced errors and so many double faults. Like, couldn't serve today. Everything together just brought me really bad result."

"I think in the beginning it was okay, but then it just got worse, like, I felt that I'm not myself today on the court," Ostapenko continued. "I was just trying to manage and fight until the last point, but she was playing very defensive, and I was just making so many unforced errors, which I normally don't do."

Read more: Ostapenko dishes on 'unbelievable pressure' after French Open defeat

As for Kozlova, she advances to the second round in Paris for the first time, and awaits the winner of Monday's match between Katerina Siniakova and former World No.1 Victoria Azarenka.

The double fault woes started early for Ostapenko, as she hit two while serving at 1-1 to hand over an early break to Kozlova. Later, at 3-2, the Ukrainian faced a break point, but erased it with a drop shot that would force an error from the Latvian -- a tactic which would pay off throughout the match for Kozlova.

Ostapenko tried to get her bearings as the first set progressed. She finally got back level at 4-4 by charging forward and slamming a backhand winner while up at the net. But that would not pay off, as that game started a run of four consecutive breaks of service, which would end with Kozlova back ahead at 6-5 after smacking a forehand service return winner down the line.

Kozlova ended the streak of breaks by serving out the set for a 7-5 lead, and Ostapenko was one set away from making ignominious history, after more than doubling Kozlova’s unforced errors in the opening frame (29 for Ostapenko versus 14 for Kozlova).

The players exchanged breaks twice in the opening stages of the second set, moving to a 3-3 deadlock. In that game, two more double faults by Ostapenko gave Kozlova double break point, but the Latvian fought those off with a service winner and a backhand winner respectively. But a forehand error at deuce gave Kozlova another break point, and Ostapenko double faulted for a third time in the game.

Now up a break at 4-3, Kozlova did not look back as her first French Open match victory was in sight, and over a highly-ranked star to boot. The Ukrainian was winning points via aces and volleys at this stage, and held for 5-3 by forcing another error from the defending champion with a solid backhand.

Ostapenko was unable to rebound, and two straight miscues queued up double match point for Kozlova. On the second opportunity, one final Ostapenko backhand went awry into the net, and a gleeful Kozlova tore a hole in the singles draw with by far the most famous victory of her life.