Serena Williams appeared some way below her brilliant best as she opened her Wimbledon defence with a straight-sets victory over Amra Sadikovic.

The world number one held all four slam titles after triumphing at the All England Club 12 months ago, but has since seen the US, Australian and French Open crowns slip from her grasp.

Williams was never likely to face the stiffest of challenges on Tuesday, against an unheralded Swiss opponent making her grand slam debut after reversing a previous decision to retire.

And the six-time Wimbledon singles champion will know improvement is sure to be required in future rounds, even though she was able to complete a 6-2 6-4 victory on Centre Court.

World number 148 Sadikovic appeared in danger of being humbled when, after spurning two break points in the opening game, she was ruthlessly broken to love.

However, although Williams went on to wrap up the first set with a degree of comfort, Sadikovic remained competitive and the underdog claimed a deserved break of her own - at the fifth attempt - to move 2-1 up in set two.

To her credit, the top seed got back on serve immediately, but Sadikovic looked increasingly assured as the match continued, her performance drawing appreciation from the crowd.

Williams was made to work hard, but her rival stumbled when serving to stay in the match and a backhand return winner - which was initially called out only to be overturned by HawkEye - booked the defending champion's place in round two.

"It was definitely tough, but I always expect that from everyone," Williams said on BBC Sport immediately after her victory. "I never underestimate anyone."