Britain's Elena Baltacha bowed out of the Sony Ericsson Open at the second round stage after losing 7-6 (7-4) 6-3 to Klara Zakopalova in Miami.
The 27-year-old British number one came from a set down to beat Sybille Bammer in round one but lost in an hour and 27 minutes to Czech Zakopalova, seeded 32.
Zakopalova was clinical at key moments, winning all four break point chances.
Top seed Caroline Wozniacki went through with a 6-2 7-5 win against American Bethanie Mattek-Sands.
It was a sound performance from the 20-year-old Dane, who won her 14th career title at Indian Wells last week.
She saved eight of the 10 break points she faced and although she only hit five winners there were only 15 unforced errors, while her opponent was guilty of 49.
"Especially in the first match you want to get going, you want to have the rallies," said Wozniacki, who along with all the other top seeds was given a first-round bye.
"But I really couldn't get that today, so I just had to fight. I thought I did that pretty well."
Baltacha, meanwhile, accepted that she had been outplayed by Zakopalova on the day.
"Zakopalova is a good player," said Baltacha. "She's a seed here and she's had some big wins and I knew that I had to play unbelievably well to stand a chance.
"In the first set I got myself into a tie-break despite being down a break a couple of times, but then I just gave her some loose points. After that she stepped it up."
Russian former world number one Maria Sharapova, seeded 16th and making her first Miami appearance since 2007 after battling a series of injuries, beat Croatian Petra Martic 6-3 6-2.
With Serena and Venus Williams missing the tournament because of injuries, Mattek-Sands was the highest-ranked American in the draw at number 43.
The first set of Baltacha's match was an even contest with two breaks of serve apiece before Zakopalova prevailed in the tie-break.
But while her opponent was ruthless, the Briton won only three of her eight opportunities to break the 29-year-old's serve.
Zakopalova broke twice in the second set and closed out victory in accomplished fashion.
Former US Open finalist Jelena Jankovic, the sixth seed, brushed aside Monica Niculescu of Romania 6-3 6-1 in an hour and five minutes.
Fourth seed Samantha Stosur, the beaten finalist in last year's French Open final and quarter-finalist in this event for the last two years, took just 58 minutes to advance into round three, beating China's Zheng Jie 6-2 6-1.
Among the seeded players eliminated were number 10 Shahar Peer, number 13 Flavia Pennetta and Nadia Petrova, seeded 18th.
Peer was beaten 6-2 6-2 by Anabel Medina Garrigues, Pennetta lost to Iveta Benesova 6-3 6-0, while Petrova retired because of dizziness trailing Sabine Lisicki 4-6 6-3 3-1.
Meanwhile, British men's number one Andy Murray will play Alex Bogomolov in round two on Friday after receiving an opening round bye.
American Bogomolov beat Romanian Victor Hanescu 6-3 4-6 7-5 in the opening round.
World number five Murray will be seeking to atone for his early exit in Indian Wells earlier this month when he was beaten 7-6 (7-4) 6-3 by American qualifier Donald Young.