Venus Williams was a high-profile first-round casualty at the Australian Open on Monday but the seven-time grand slam champion was happy with her performance as she heaped praise on Belinda Bencic.

Runner-up to sister Serena at Melbourne Park last year, fifth seed Venus Williams fell at the first hurdle, stunned by Switzerland's Bencic 6-3 7-5 on day one of the year's opening slam.

Fans packed into Rod Laver Arena expecting the 37-year-old American favourite to advance but she was no match for the in-form Bencic, who teamed up with Roger Federer to win the Hopman Cup.

"I think she played well. I don't think I played a bad match," said Venus after a slam was left without a Williams sister in the second round for the first time since the 2011 French Open.

"She just played above and beyond. I just have to give her credit for that."

Williams tallied 26 unforced errors to 12 and was broken five times by Bencic as the American veteran exited the tournament.

"I didn't play so bad, I wasn't missing every shot," she said. "Just didn't work out."

Bencic, who had been plagued by injury before returning to full fitness in 2017, added: "It obviously means a lot. It was amazing, this match. I think the level was great. She beat me four times before, so I was really happy, took my chance this time.

"Just happy to be back playing. It was a very, very long time when I was out. I couldn't wait to come back, so it means a lot."

Williams was not the only notable American to depart on Monday following the elimination of reigning US Open champion Sloane Stephens.

Stephens - seeded 13th - was upstaged by Zhang Shuai 2-6 7-6 (7-2) 6-2 as she was left searching for her first win since reigning supreme at Flushing Meadows last September.

"Even though I lost, I'm not too sad," Stephens told reporters. "Oh, my God, no. Everything is good.

"Relax, everybody. It will be okay. Don't worry. We will get back to having fun soon. Just give me a little bit to regroup and we will be okay."