Naomi Osaka says thinking about retaining her number one world ranking hindered her in her Madrid Open quarter-final loss to Belinda Bencic.
Osaka led by a break in the final set and served for the match, but Bencic fought back to win 3-6 6-2 7-5.
Romania’s Simona Halep could replace Osaka at the top of the rankings if she wins the title in Madrid.
“People have been telling me if I get to the semis I can stay number one, so I wanted to do that,” Osaka said.
“I wanted to do that and I was thinking about that while I was playing, which wasn’t necessarily a good thing.”
Osaka secured the number one ranking when she won her second Grand Slam title at the Australian Open in January.
“I play the best when I’m calm and today I wasn’t calm, so that is the biggest thing I can learn,” she said.
Bencic, who ended Osaka’s Indian Wells title defence in March, produced some fine returns to come through a tight final set.
Top seed Osaka broke in the fifth game of the deciding set and held off two break points in her next service game to retain her advantage.
However, she faltered as she served for the match at 5-4 and Bencic broke to love to level the set at 5-5.
The Swiss, who won the Dubai Championships in February, then won the next two games to close out an impressive win.
The 21-year-old said she had “so much drama in my head” during the three-set defeat.
“I wanted to win this match more because she beat me already before and I just didn’t want to lose twice in a row,” Osaka added.
French Open champion Halep earlier beat Australian ninth seed Ashleigh Barty 7-5 7-5 to set up a meeting with Bencic.
America’s Sloane Stephens, who lost to Halep at Roland Garros last year, overcame Croatia’s Petra Martic 6-4 6-3.
Kiki Bertens of the Netherlands ended Petra Kvitova’s title defence in a rematch of last year’s final.
Bertens overpowered the Czech 6-2 6-3 and will play Stephens next.