LONDON — U.S. Open champion Sloane Stephens tumbled out of Wimbledon on the first day Monday, looking rusty and out-of-sorts in her 6-1, 6-3 defeat by Croatia’s Donna Vekic.

The 25-year-old fourth seed, who played no grass warm-up tournaments after reaching the final of the French Open on clay, looked uncomfortable on the fast surface, moving awkwardly and failing to find the lines.

Vekic, winner at Nottingham last year and a semi-finalist there last month, looked more sure of her footing and aggressive, serving powerfully and forcing her opponent into errors. After breaking serve twice, the Croatian needed only one set point and 25 minutes to wrap up the first set.

Stephens showed some fight at the start of the second set, winning the first two games to love with some powerful serve-and-volley tennis, but she could not keep the rhythm going.

Vekic, who played in three grasscourt tournaments coming into Wimbledon, won the next three games, moving nervelessly along the baseline and salvaging less-than-pinpoint serves with bullet-like returns.

Vekic’s own demon — the double fault — came back to haunt her in the sixth game. She served two in the next game out of a total of nine in the match, to offer Stephens a lifeline.

But the listless American could not take it. Her shots continued to fly wide and long off her racket, and she produced 26 total unforced errors in the match.

Vekic earned her first match point in the ninth game, when Stephens hit the net. Stephens saved that with a blistering crosscourt forehand and then successfully defended another two.

She finally succumbed by dumping another forehand into the net on the fourth match point.

“I tried to be aggressive,” Vekic said of her game plan. It was the 22-year-old’s biggest victory of her career.