Sinner has been on a roll this season, winning the Australian Open for what is expected to be the first of many Grand Slam titles
The American, who suffered a knee injury after beating Justine Henin to lift the Australian title in Melbourne after a three-set battle, looked listless in the early stages of her second-round match and struggled on serve.
Bacsinszky capitalised, mixing things up nicely with her drop shots especially effective against a leg-weary Serena and unleashing eye-catching winners from both sides to break twice and race 4-1 ahead on the red clay surface.
Williams clawed her way back, despite a few erratic moments, saving three set points on her opponent’s serve before dominating the tiebreak and the second set to book a meeting with Germany’s Andrea Petkovic.
“I got off to a bit of a slow start but it was good I was able to fight it off,” the 12-times singles grand slam winner told a news conference at the warm-up for the French Open.
“I don’t think anything went wrong. I thought she played excellently. She was really playing well so I just had to pick up the level of my game.”
“I hope by the time I get to Roland Garros, I’ll be playing really well. It (Tuesday’s match) is definitely a start.”
Safina ousted
Russian holder Dinara Safina, also plagued by injury problems, slumped to a 6-4 6-7 6-1 defeat by world number 43 Alexandra Dulgheru of Romania.
Third-seeded Safina, who has missed most of the season with a back injury, made a sluggish start on clay made heavy by rain and trailed 4-0.
She levelled at 4-4 but surrendered the initiative immediately, skewing an easy volley wide to lose serve on the way to conceding the first set.
She eventually claimed a topsy-turvy second set on a tiebreak before crumbling in the decider.
“I’d been two months completely without touching the racquet and simply not doing any practice or preparation,” Safina said. “I practised just a little bit and obviously this was very disappointing.
“The only good thing about this match was that my back didn’t create any problems.”
Fourth seed Venus Williams, also back from injury, reached the third round with a 6-2 6-2 win over Swiss Patty Schnyder.
Denmark’s world number two Caroline Wozniacki dropped just one game in her win over Italian wild card Maria Elena Camerin.
Twice champion Jelena Jankovic of Serbia, seeded seven, overcame American Bethanie Mattek-Sands 6-3 3-6 6-1.
Her compatriot Ana Ivanovic, who has slipped to 58th in the rankings after being top two years ago, showed she might be on her way back to form by beating ninth seed Victoria Azarenka 6-4 6-4 to set up a clash with world number six Elena Dementieva.
Sinner has been on a roll this season, winning the Australian Open for what is expected to be the first of many Grand Slam titles
The Australian Open champion will move to a career-high second in the ATP world rankings after winning the third title of the season
Zverev's win marked his return to the Miami Open semifinals for the first time since his runner-up finish in 2018
The victory marked the first time Collins has won five consecutive matches on the WTA Tour since the 2022 Australian Open
The Serb had brought Croatia's 2001 Wimbledon champion on board as a member of the coaching team ahead of the grass court Grand Slam in 2019
Playing in her home tournament in southern Florida, Gauff fell 6-3 1-6 6-2 to Frenchwoman Garcia in the fourth round on a day of upsets
The Belarusian, whose former boyfriend died on Tuesday, vented her frustration by smashing her racquet to pieces after losing the match
Halep looked terrific in the first set, then encountered trouble later