Cristiano Ronaldo was controversially denied an injury-time winner as Portugal blew a two-goal lead to be held by Serbia in World Cup qualifying on Saturday, while Romelu Lukaku's 59th international goal was only enough to salvage Belgium a 1-1 draw with the Czech Republic.

Portugal appeared to be cruising to three points in Belgrade when leading 2-0 at the interval through Liverpool winger Diogo Jota's first-half double.

Nemanja Radonjic's half-time introduction proved crucial for Serbia, though, as he set up Aleksandar Mitrovic and Filip Kostic to net in the first 15 minutes of the second period.

Portugal had to settle for a 2-2 draw, with Ronaldo left seven short of Iranian Ali Daei's all-time record of 109 international goals after failing to score for a second straight match.

He appeared to have netted in the 93rd minute, but without Video Assistant Referee or goal line technology in use, officials decided that Serbia defender Stefan Mitrovic had cleared the ball before it crossed the line.

To add insult to injury, Ronaldo was booked for his protestations.

"We scored a goal that was not given when the ball went in," Portugal coach Fernando Santos told the country's public service broadcasting media RTP. "In a match of this level, that isn't possible."

Belgium, the world number one-ranked side, sit behind the Czech Republic on goal difference in the early Group E standings.

Lukas Provod put the Czechs ahead shortly after half-time in Prague with a magnificent long-range strike to score his first international goal.

But 10 minutes later, Lukaku collected Kevin De Bruyne's pass, turned his marker and drilled the ball under home goalkeeper Tomas Vaclik to score for the 19th time in his last 16 games for Belgium.

Roberto Martinez's men almost took the lead 14 minutes from time, but after Lukaku teed up De Bruyne, the Manchester City midfielder smashed a low shot against the post.

"If you look at Czech Republic's results these last months, we knew that it would be a tough game," Belgium captain Jan Vertonghen told the country's broadcasting media VTM News. "They created the good chances, won almost every second ball. But at home we are stronger."