Naomi Osaka lost her first match since becoming world number one as she was beaten 6-3 6-3 by Kristina Mladenovic in the Dubai Tennis Championships.
Japan’s Australian Open and US Open champion Osaka, 21, was playing for the first time since splitting with coach Sascha Bajin.
She lost in 66 minutes against the world number 67 in the second round.
They traded six successive breaks of serve before France’s Mladenovic, 25, held when serving for the match.
Osaka received a bye in the first round.
Mladenovic, who had not won a match this year before the tournament, will play Spain’s Carla Suarez Navarro in the third round.
Second seedand two-time Wimbledon champion Petra Kvitova fought back from a set down to beat fellow Czech Katerina Siniakova 6-7 (3-7) 6-4 6-4 and reach the last 16.
Germany’s three-time Grand Slam champion Angelique Kerber saved two set points in the opener before winning 7-6 (7-4) 6-3 against Slovenia’s Dalila Jakupovic.
Russia’s Viktoria Kuzmova overcame seventh seed Kiki Bertens 6-2 4-6 7-6 (8-6), while American Sofia Kenin progressed with a 6-3 2-6 6-4 victory at the expense of Russian 11th seed Daria Kasatkina.
Taiwan’s Hsieh Su-wei saw off Belarus’ Aliaksandra Sasnovich 6-1 6-2.