JOIN USJapan's Kei Nishikori lost his opening match of the clay season on Wednesday, with Asia's top player crashing to a 7-5, 6-4 defeat to Pierre-Hugues Herbert at the Monte Carlo Masters.
Fifth seed Nishikori reached the final here last year but will lose a chunk of ranking points after he hit 25 unforced errors and just 14 winners in succumbing to Frenchman Herbert.
Nishikori will now move on to Barcelona next week, where he owns two titles as he works on his clay form as the run-up to Roland Garros intensifies.
Herbert, who overcame Fernando Verdasco in the first round, struck 31 winners in his upset to avenge a three-set loss to Nishikori in Rotterdam earlier this season.
"I'm very happy to finally beat Kei," the 49th-ranked Herbert said. "I was very confident with my defence.
"I came to the match with good feelings after the first round."
Nishikori lost the Monte Carlo final a year ago to 11-time champion Rafael Nadal.
Alexander Zverev rediscovered traces of his best form with a 6-1, 6-4 defeat of Felix Auger-Aliassime.
The German reached the third round as he tamed the 18-year-old who broke through for a Miami semifinal last month.
Rafael Nadal starts his bid for a 12th title here later, with an opening match in the second round against fellow Spaniard Roberto Bautista Agut.
The 21-year-old Zverev played the semifinals in Monte Carlo a year ago but is yet to hit his stride this season.
That may be starting to change.
"This is the first time this season I've felt really healthy," said Zverev, who lost prematurely at both Indian Wells and Miami, as well as in Marrakech last week.
"I've been training well but haven't been able to take that into matches.
"I live just 500 metres from here, I'm comfortable on this court. I'm starting to feel better about my game.
"I'm really ready for the clay season."
Canadian Auger-Aliassime has made a fast rise in the rankings from 175 a year ago to his current 33rd.
Zverev swept the opening set in 30 minutes but had to get through a series of three straight breaks of serve between him and his opponent before finally advancing in 78 minutes.
The third seed broke six times and will next face Italy's Fabio Fognini, who reached the third round when French opponent Gilles Simon was unable to play their match due to back problems.
Greek sixth seed Stefanos Tsitsipas moved ahead, defeating Mikhail Kukushkin 6-3, 7-5.
"I need to maintain my focus, my concentration, my aggressive mindset when I'm on the court," Tsitsipas said.
Britain's Cameron Norrie defeated Marton Fucsovics of Hungary 7-6 (7/3), 6-3.