Indian Wells - World number three Karolina Pliskova battled back to beat Olympic champion Monica Puig 1-6, 6-4 6-4 on Friday, but says she'll have to do better to go further at Indian Wells.
"I don't even think I deserved to win tonight," third-seeded Czech Pliskova said after recovering from a break down in the third set to reach the third round.
Puig, ranked 43rd in the world, had never taken a set off Pliskova in three prior meetings.
But she was on fire from the baseline early, breaking the big-serving Czech four times in the opening set.
Pliskova, winner of titles in Brisbane and Qatar already this season, was in trouble again in the decisive set, down one break at 0-3
But Pliskova won six of the last seven games to advance despite 34 unforced errors to just 19 winners.
"Hopefully I can get something from it and move on and play better next time," said Pliskova, who is aiming to improve on her semi-final run at Indian Wells last year.
In other second-round matches, 17-year-old American Kayla Day shocked Australian Open semi-finalist Mirjana Lucic-Baroni to earn a clash with French Open champion Garbine Muguruza.
Day out-lasted the 35-year-old Lucic-Baroni 6-4, 5-7, 7-5, rallying from a break down in the third to advance.
Lucic-Baroni served for the match in the final set, but the young American, who won the US Open junior title last year, kept her nerve.
"It was a really close match throughout the entire match. I think I started to really believe I could win at 6-5 in the third. It was still a really tough game, because I was a little bit tight to close it out. But my serve had been giving her a lot of problems, so I knew if I stuck to my game it might work out."
Day was looking forward to the challenge offered by seventh-seeded Muguruza, a 6-2, 6-3 winner over Belgian veteran Kirsten Flipkens.
Muguruza was firmly in control having raced to a 5-0 lead in the second set before Flipkens began to make inroads on the Spaniard.
"It's always difficult to close out your first match," Spain's Muguruza said. "She's a very talented player, unbelievable, with a very different game. I got nervous!"