Naomi Osaka's quest for a third consecutive Grand Slam title collapsed as she lost 6-4 6-2 to Katerina Siniakova on Suzanne-Lenglen. The top-seeded Osaka was beaten by Czech Siniakova, the worldвЂ™s top-ranked doubles player, 6-4 6-2 on a hot day at the Court Suzanne-Lenglen at Roland Garros. Osaka, who has yet to reach the second week in four appearances in Paris, got broken three times and failed to take her chances. Despite entering Roland Garros as world No. 1 for the first time, Osaka was taken to three sets in her first two matches. She rallied back to beat world No. 90 Anna Karolina Schmiedlova of Slovakia in the first round after losing the first set, and recovered from a set and a break down in a second-round clash against fellow double Grand Slam winner Victoria Azarenka of Belarus. With back-to-back titles at the US Open and the Australian Open, the 21-year-old from Japan had been trying to do something not even the greatest players in tennis history have done: winning their first three Grand Slam singles title in a row. The 42nd-ranked Siniakova, the reigning French Open and Wimbledon doubles champion, showed off her doubles credentials in the opening set, unsettling her opponent with sliced forehands and drop shots. Breaking serve at 4-4, Siniakova clinched the first set in 44 minutes on her second set point as Osaka dumped a forehand into the net. Osaka struggled to settle in the second set, going down a double break with a double fault. Serving for her first win over a world No. 1 at 5-2, Siniakova kept her nerve and won the match on Osaka's 38th error. "It is incredible, it is something I can't believe, I'm so happy," Siniakova said after reaching her first last-16 round of a major for the first time. "Roland Garros brings out my best and I hope it will continue," she added. "I just feel like there has been a weight on me," Osaka told reporters after the Czech ended a 16-match win streak in the grand slams. "That's because everything is sort of new. I have played the French Open before but not in this circumstance or situation, whichever one." Hugely popular in her native Japan with a string of lucrative corporate endorsements, Osaka has been getting used to her new status. "This tournament I have had a feeling that was different to the other Grand Slams, or every other Grand Slam that I have played, because usually I find it very freeing and fun, and this time around I was kind of tense the entire time," she said. "I would just say I'm very disappointed in how I played, and I wish I could have done better," said Osaka. Osaka squandered all seven break points she created in the first set. "My returns weren't great at all today," she said. "I knew that I was flat. I knew that if I stayed in the rallies on the forehand side, I would probably definitely, 99% of the time, win the rally. But for some reason I kept switching down the line and doing really weird stuff." Still, Osaka said she is leaving Paris with a positive mindset. "It's weird, but I think me losing is probably the best thing that could have happened," she said. "I was overthinking this calendar slam. For me this is something that I have wanted to do forever, but I think I have to think about it like if it was that easy, everyone would have done it. I just have to keep training hard and put myself in a position again to do it, hopefully."