Lech raced into a two-goal lead after Rudnevs fired home a penalty before scoring a second on the half-hour mark.
Juve battled back, however, led by their influential veteran striker Alessandro Del Piero who twice set up Giorgio Chiellini either side of half-time to draw his side level.
Del Piero then fired the Old Lady ahead with a marvellous long-range strike, but that goal was bettered in injury time when Rudnevs blasted an equaliser from 35 yards to complete his hat-trick and steal a point for his side.
Juventus were expected to make light work of a Lech side who had missed out on the Champions League after losing their play-off to Sparta Prague.
It was the visitors, however, who made all the early running with Rudnevs trying his luck from distance before his side won a 12th-minute penalty.
Felipe Melo fouled Slawomir Peszko in the area and Rudnevs placed the spot-kick into the bottom-right corner to give his side an unlikely lead.
They then doubled their advantage after 30 minutes when the ball broke for Latvia international Rudnevs to shoot low into the left corner.
Juve were left shell-shocked but they grabbed an important goal in the shadows of the break when Del Piero crossed for Chiellini to head home from six yards.
Chiellini then levelled matters with his second three minutes after the break when Del Piero's corner picked him out to fire home from close range.
Del Piero, who turns 36 in November, was at his influential best as he fired a free-kick just over before unleashing a super 30-yard drive after 68 minutes to send his team ahead.
The veteran's strike looked like it would put the seal on a nervy night for Juve, but there was one last sting in the tail.
With the match in injury time Rudnevs' ambitious drive from 35 yards flew past Alex Manninger to hand Lech a deserved share of the spoils.
Source: DSG
