"I think she's doing everything great", Serena said, according to WTA Tennis.
Gauff showed fight despite the straight-sets loss. Gauff hit three forehand errors in the first game of the match to be broken immediately, yet broke back and held lead 2-1.
Gauff received a kiss for good luck from her father, Corey, as she walked out onto Court One shortly before 2.30pm. Halep responded by winning the next three games before Gauff staved off two match points to force a ninth game.
The match between Coco Gauff and Simona Halep will follow, while number 16 seed Johanna Konta and 2014 victor Petra Kvitova are the only two players still in the ladies' tournament to play on Centre Court today.
She'll move on to face Shuai Zhang in the quarterfinals later this week.
Karolina will start her Wimbledon campaign today against China's Zhu Lin and her sister will play her opening round match on Tuesday against Slovenia's Kaja Juvan.
The 15-year-old Gauff, from Delray Beach, Fla., was the youngest player to qualify for the Wimbledon main draw in the modern era.
Barty and Serena Williams dominate to enter Wimbledon fourth round
Knee problems that have plagued her season have yet to resurface in London , which bodes well for the challenges ahead. Serena broke in the sixth game, though, and clinched the set at the third time of asking by making a forehand victor .
From then on Gauff seemed to lose confidence as she lost the first set 6-3 after 41 minutes.
The youngster lost the first set as she appeared unable to cope with the nerves in front of the 15,000 fans.
Williams reached her 14th Wimbledon quarterfinal with a 6-2, 6-2 win over Carla Suarez Navarro. However, action on Centre Court and Court One doesn't begin until 1 p.m. local time (8 a.m. ET), with Serena Williams kicking off proceedings on the latter. This is the only major tournament that schedules the middle Sunday off - and then the only one to play every singles match the following day.
If she reaches the quarter-finals, Gauff stands to earn £294,000, not to mention millions more in sponsorship and endorsements as her status grows.
The 15-year-old has emerged as tennis's new sensation and admitted her new-found celebrity had left her star-struck. Number six seed and 2014 victor Petra Kvitova takes on Britain's Johanna Konta on Centre Court, while Barbora Strycova faces number 21 seed Elise Mertens.
"This is one of my favourite tournaments to play".