Germany's Angelique Kerber shook off last weekend's Fed Cup trauma with a 6-3 6-2 first-round victory over Petra Kvitova at the Porsche Grand Prix in Stuttgart.
Kerber had lost in straight sets to Kvitova and Karolina Pliskova as the Czechs won the Fed Cup semi-final, but was reborn on Wednesday on the duel of lefties and two-time grand slam winners.
Fired up by a noisy home crowd, Kerber wobbled on her first two service games but then grew in confidence and took the last four games from 3-2 down to win the set with two breaks on a forehand error from Kvitova.
Kerber saved a break point in the opening game of the second, broke for 3-1 and wrapped up matters after 1 hour 17 minutes on third match point on another Kvitova error after the Czech eighth seed saved the first two with a forehand and service winner.
"It wasn't easy because I lost an important match to Petra on Sunday. But I learnt my lesson. I tried to be more aggressive and dictate play," the 12th-ranked Kerber said.
"When I came out I had goosebumps. You don't play at home so often, I love playing here," the 2015 and 2016 Stuttgart champion added.
Kvitova said: "I wish that nothing would have changed. But she played much better today."
Stephens meanwhile crashed in 56 minutes against the wildcard entry Vandeweghe, winning only the opening game and dropping all her serves in a performance which drew some boos from the crowd at the last changeover and lukewarm applause at best when she fled off court afterwards.
CoCo Vandeweghe. Source: AAP
"I probably played OK," Vandeweghe said. "I didn't really do all that much, she gave me the errors."
Stephens last won a match outside North America at Wimbledon 10 months ago and has gone out in the first round in seven of her 10 tournaments since her US Open title in September. Last month's Miami title did not lead to a turnaround either.
Vandeweghe now faces German title holder Laura Siegemund Thursday while Kerber plays Estonian Anett Kontaveit, who denied last year's runner-up Kristina Mladenivic serving out the match at 5-4 in the third set to win 5-7, 7-6 (7-3), 7-6 (7-5) in 3:09 hours.
The third American at the event, losing US Open finalist Madison Keys, went out against Russian Anastasija Pavlyuchenkova in a 2:45-hour marathon, 7-6 (9-7), 5-7, 6-4, after wasting a set point in the first set.
Qualifiers Veronika Kudermetova of Russia and Zarina Diyas of Kazakhstan advanced while world number one Simona Halep of Romania was to open the second round against Slovakia's Magdalena Rybarikova in the late match.
