Naomi Osaka lost the second match of her comeback at the Brisbane International to Karolina Pliskova, another former world number one.
The Czech player, a three time champion at the tournament, won 3-6 7-6 (7-4) 6-4 in the Pat Rafter Arena.
Japanese Osaka, who gave birth to a daughter in July, was playing her first tournament for 15 months.
"The week is shorter than I wanted it to be, but it was a great match and I had fun," the 26-year-old said.
"I think I relearned that - this is my opinion - but I think I have a lot of heart. I think when I'm playing and I'm at my best, I'm just really putting my entire soul into every point."
Osaka, a four-time Grand Slam champion said in a BBC interview before the tournament that she thought defeats may sting less now her daughter Shai is the centre of her life.
"I do feel different," she continued.
"Of course I feel sad, but the sadness is me being, like, I wish I could have done better, because I know I'm spending so much time away from her, so I want it to be worth it somehow.
"Also, I feel like it fuelled me a lot to do better. It does sting a little bit less because I know that I literally gave everything that I could."
Pliskova won just one more point than Osaka in an incredibly tight match in which she saved 10 of the 12 break points the four time Grand Slam champion created.
Osaka started well and took a fifth opportunity to break for a 2-0 lead following some fine passing winners and three double faults by Pliskova.
The currently unranked Japanese handed back the advantage with a double fault in the very next game but proved too consistent for Pliskova after that, and took the first set 6-3.
The world number 39 found herself in trouble at the start of the second set but having recovered from 0-40 down to hold serve in the third game, started to play significantly better.
She rediscovered her formidable serve and the set went to a tie-break, which Pliskova played flawlessly and won 7-4.
The third set was extremely close. All three of Osaka's break points were saved by Pliskova, who secured the decisive and sole break in the third game. A series of fine serves in the final game ensured there was no way back for Osaka.
But the two matches in Queensland should give her plenty of encouragement. Osaka produced two high qualify performances - moving well and striking the ball powerfully and consistently.
Osaka, a two-time Australian Open champion, will now head to Melbourne where on Tuesday she will open a week of charity matches on the Rod Laver Arena as a curtain raiser to the first Grand Slam of the year.
BBC