"I am sure he will bounce back quickly. The Australian Open is his tournament and I am almost convinced that he will win a record-breaking 21st slam in Melbourne."
Barty used her all-court game, full of all manner of spins and angles, to great effect as she started and finished the match in dominant fashion.
"She's a fighter, a great competitor and she will be sorely missed," Barty, who showed no sign of the hip injury that forced her to retire hurt at the French Open, said on court.
The Court Three clash of veterans, one of several held over due to rain on Monday, was the 271st victory of a remarkable Grand Slam career for Williams and, despite serving 10 aces, did not come easy.
"I probably would have taken the place I am today," the 39-year-old, who pulled out from the French Open last week to protect his body after a gruelling third-round win.
"The knee just needs always extra care, and I hope it's also going to react well to the grass," he said, adding that he feels "all my strengths get amplified" when he plays on grass.
World number one Djokovic received a standing ovation from the home crowd by the Danube river in the Serbian capital after an erratic display against world number 255 Molcan, who battled through from the qualifying rounds.
Djokovic also lost to Russian Aslan Karatsev in the Serbia Open semi-finals before being defeated by Rafael Nadal in the Rome Masters final and his display against Molcan was at times unconvincing.
"After having some problems in my left foot for a while, I have decided to do a small procedure on it," Wawrinka said in a tweet accompanied by a picture of him on a hospital bed.
"Everything went well. I will be out for a few weeks but can't wait to start practising again soon."
Unfortunately I am not feeling 100% in my lower back, and I have made the tough decision to withdraw from the 2021 Dubai Duty Free Tennis Championships.
"I have great memories from winning my 20th WTA title after some really tough matches in Dubai last year so I'm disappointed that I won't be able to defend my title."
The 20-time Grand Slam winner has been struggling with a back issue since the build-up to the Australian Open and was forced to skip Spain's ATP Cup ties earlier this month.
"Once I got back to Spain I visited my doctor and together with my team they've advised not to play this upcoming week."