Serena Williams say she has "a long way to go" on her comeback after the birth of her child following defeat by sister Venus in round three at Indian Wells.
The former world number one hit four aces but had her serve broken four times in a 6-3 6-4 loss in California.
"It wasn't easy, obviously," said Serena, who was in her first event since beating Venus in the 2017 Australian Open final.
Venus moves on to the round of 16 where she will face Anastasija Sevastova.
Latvian Sevastova beat German Julia Goerges in straight sets 6-3 6-3. It was the 29th meeting between the Williams sisters - Serena now leads 17-12 - and it lasted 87 minutes in front of a main stadium crowd of more than 10,000 fans, including current world number one Simona Halep.
Serena, 36, saved a match point against her elder sister but hit a forehand long to end the contest.
"It was good to play and try to get in the rhythm and get into the swing again," added Serena, who gave birth just over six months ago.
"I can't really replicate the situation no matter how much I do in practice. I make those shots 10 times out of 10 in practice.
"It's just the nerves, the anticipation you feel naturally. It's a little bit of everything that comes in a match that just doesn't normally happen.
"It's good that I don't have to say that this is the best tennis I have ever played and I lost. My room for improvement is incredible," added Serena.
"So I have just got to keep saying at each tournament that my goal is just to be better than the last. I don't want to go backwards."
Serena was already pregnant with daughter Alexis Olympia, who was born on 1 September, when she beat Venus 6-4 6-4 in Melbourne at the start of 2017.
However, she revealed in February that she "almost died" after giving birth and was bedridden for six weeks.