Petra Kvitova believes her desperation for a third Wimbledon title was a telling factor as she suffered a shock first-round loss.
The Czech, a pre-tournament favourite with bookmakers and many experts, was stunned on Court One as Aliaksandra Sasnovich won 6-4 4-6 6-0 to produce the biggest shock so far.
Kvitova has been prolific on the WTA Tour, winning five titles this year, but is struggling to make the same impact at grand slams.
“When I was kind of younger, I played better in the grand slams than the other tournaments. Now is the time when I’m playing better in the other tournaments than the grand slams.”
Maria Sharapova’s Wimbledon return ended in defeat to fellow Russian Vitalia Diatchenko.
It was also her first appearance on grass in that time, having pulled out of competing in Birmingham last month.
The 2004 Wimbledon champion won the first set on a tie-break and led 5-2 in the second but Diatchenko broke back, took the tie-break and prevailed in the third to win 6-7 (3/7) 7-6 (7/3) 6-4.
Garbine Muguruza said it was business as usual after getting the defence of her title off to a winning start against Naomi Broady.
The Spaniard was the queen of Centre Court in 2017, beating Venus Williams in the final, and she enjoyed her return there with a 6-2 7-5 success over the British number four.
Muguruza is not weighed down by the prospect of defending her title, insisting it has made her more confident.
“I don’t feel different,” she said. “I do feel more confident knowing that I won Wimbledon.
“As a tennis player, when you win Wimbledon, I think it’s winning the best tournament. Your mind is like, ‘I won Wimbledon, priceless’.”
Romanian Halep was playing her first match since winning the French Open last month – her maiden grand slam title and she might fancy another here.
Seventh seed Caroline Garcia was the victim of another upset after she was downed 7-6 (7/2) 6-3 by Belinda Bencic, while Angelique Kerber got her campaign off to the best possible start with a straights-sets win over Vera Zvonareva.
Dominika Cibulkova put a seedings disappointment behind her by beating Alize Cornet.
Cibulkova strongly contested the decision but let her tennis do the talking against Frenchwoman Cornet, winning 7-6 (7/4) 6-1.
There were also wins for Jelena Ostapenko, Carla Suarez Navarro, Naomi Osaka and Eugenie Bouchard.