Williams, 40, was playing competitive singles tennis for the first time since retiring hurt on Centre Court a year ago.

The seven-time Wimbledon champion lost the opening set but, roared on by a partisan crowd, won the second and twice went a break up in the third.

France's Tan was playing in the Wimbledon main draw for the first time, but demonstrated admirable mental strength to come back and the match went to a third-set tiebreak.

The 24-year-old lost the opening four points but again rebounded, ultimately winning it 10-7 and the match 7-5,1-6, 7-6.

Williams, the American great who has won 23 Grand Slam singles titles - one shy of the all-time record - will no doubt face questions about her future following the gruelling defeat in three hours and 10 minutes.

Świątek, the top seed, brushed Croatia's Fett aside 6-0, 6-3.

Dutch player Lesley Pattinama Kerkhove awaits the Pole in round two, while Tan is set to play Spain's Sara Sorribes Tormo.

Chinese teenager Zheng Qinwen beat Grand Slam winner Sloane Stephens of the United States in straight sets, but fellow American Coco Gauff fought back from a set down to beat Elena-Gabriela Ruse from Romania.

Maria Sakkari, the Greek fifth seed, was a straight-sets winner, so too Canada's one-time US Open champion Bianca Andreescu and Spain's fourth seed Paula Badosa.

In the men's singles, sixth seed and last year's quarter-finalist Felix Auger-Aliassime from Canada went down 6-7, 6-4, 7-6, 7-6 in a shock defeat to American Maxime Cressy.

Perhaps of more significance, Matteo Berrettini, the Italian who was Wimbledon runner-up in 2021, had to pull out after testing positive for COVID-19.

Danish teenager Holger Rune, who reached the last eight at the French Open in May, lost in straight sets to US player Marcos Giron.

Rafael Nadal's quest for a record-extending 23rd men's singles Grand Slam victory and third of this year began with a four-set triumph against Francisco Cerúndolo.

Despite breaking Nadal's serve on four occasions - including at the start of the pivotal fourth set - Cerúndolo lost 6-4, 6-3, 3-6, 6-4.

Greece's Stefanos Tsitsipas, seeded fourth, also won in four sets, versus Swiss Alexander Ritschard in his case.

Wimbledon is taking place without Russian and Belarusian players as they are banned because of the war in Ukraine - a decision which saw the event in London stripped of its ranking points.