Venus Williams, 33, hasn’t won a Grand Slam since her 2008 Wimbledon title, but she looked like the Venus of old Monday, upsetting No. 12-seeded Kirsten Flipkens of Belgium 6-1, 6-2.

NEW ORLEANS — Venus Williams, at 33, has been fighting a bad back this year and will cope for the rest of her life with Sjogren’s Syndrome, the energy sapping condition that forced her out of the 2011 U.S. Open.

But watching her yesterday on Ashe Stadium Court, there was little question that when’s she healthy, she dangerous. Just ask her opponent, the well-beaten No. 12 seed Kirsten Flipkens.

“Anybody who knows anything about tennis could see she was a top 10 player out there today,” said Belgium’s Flipkens, who was trounced 6-1, 6-2. “This is her play garden, Arthur Ashe. She was on fire.”

The two-time U.S. Open champion and seven-time Grand Slam winner evoked memories of her dominant years, the years before sister Serena surpassed her, the years before injury and illness took a toll on her talent.

But nothing seems to take a toll on her confidence, despite being ranked No. 60.

“I stay positive because I know I can play great tennis,” said Williams, who won a small tournament in Luxembourg last year, her first victory since 2010. The last of her Slam victories came at Wimbledon in 2008. “Sometimes you have to have losses. When I had losses, it always motivates me a lot to do better and to work harder.”

A bad back has made it particularly difficult to work harder on anything. “I’ve been without a serve this season,” said Williams. “I think I was serving in the 40 percentages this summer, so it was challenging.”

Her sister, Serena, dominated Monday night with a 6-0, 6-1 win over Francesca Schiavone, who won the 2010 French Open and has been a quarterfinalist at the U.S. Open. She’s been ranked as high as No. 4 but is 54th this week.

“I knew playing a former Grand Slam champion in the first round was a really, really tough draw,” No. 1-ranked Serena said, “so I tried to be super serious.”

All told, the match only took an hour and ended as a light rain began to fall. Play was suspended, and the last scheduled match of the night session, 17-time major champion Roger Federer vs. 62nd-ranked Grega Zemlja of Slovenia, was postponed until today.

On the men’s side, Spain’s Rafael Nadal improved to 16-0 this year on the hard surface, defeating American Ryan Harrison 6-4, 6-2, 6-2 in the first round.

The victory marked the first for No. 2-seeded Nadal on any Grand Slam surface other than clay since the first round of Wimbledon in 2012.He won titles at Montreal and Cincinnati in the lead-in to the U.S. Open.

“We go day to day,” he said about his chances here.