He featured regularly during the 1999–2000 season as Ipswich challenged for promotion to the Premier League. He made his first appearance of the season on the opening day, featuring as a second-half substitute in a 3–1 home win over Nottingham Forest at Portman Road. He made 40 appearances in all competitions over the course of the season as Ipswich once again finished 3rd in the First Division, qualifying for the play-offs as a result. Ipswich faced Bolton Wanderers in the play-offs for the second consecutive season. Wilnis started the first leg of the play-off semi-finals away at the University of Bolton Stadium, with the match ending in a 2–2 draw after Ipswich came back from 0–2 down following a brace from Marcus Stewart. He did not feature in the semi-final second leg which Ipswich won 5–3, making the aggregate score 7–5 to earn Ipswich a place in the play-off final at Wembley Stadium. Wilnis featured as a late substitute during the final, replacing Jermaine Wright in the 89 minute. Ipswich won the final 4–2, earning promotion to the Premier League for the 2000–01 season.