British number one Katie Boulter was knocked out of the US Open second round in a "horrendous" straight-set defeat by Spain's Jessica Bouzas Maneiro.

Boulter, the 31st seed, started strongly in the opening set, breaking first before Bouzas Maneiro found her footing.

The world number 74 stormed into the lead in the second set and held off a late surge to close out a 7-5 7-5 victory.

Boulter was heard saying "everything was horrendous" to her team at the side of the court.

The 28-year-old has leapt up the rankings after a strong 18 months, but she has yet to reach the second week of a Grand Slam.

Coming into the US Open, Boulter admitted she was not as prepared as she might have been for the hard courts after playing at the Olympics on the Paris clay.

She had just three completed matches on hard courts before starting her campaign at Flushing Meadows.

Meanwhile, Bouzas Maneiro was ranked outside the world's top 150 at the start of the year and had played just one Grand Slam main draw match.

The 21-year-old has hit an impressive run of form, beating defending champion Marketa Vondrousova on the way to reaching the third round of Wimbledon before retiring injured against eventual winner Barbora Krejcikova.

She will play sixth seed Jessica Pegula or former Australian Open champion Sofia Kenin in the third round, who play later on Thursday.