In the second set there was nothing to split the two teams until Sousa made a mistake at 5-6 in the tiebreak and Klaasen did likewise in the third set in the tiebreak.
In the fourth set tiebreak Venus and Klaasen fell behind early and weren't able to recover, losing it 7-3.
So we were into the fifth set and the drama happened when Sousa was broken in the sixth game.
Venus served in the next game and fell behind 0-40, allowing the Portuguese/Argentinian team to get tantalisingly close to being back in the match.
But Venus won every remaining point in the set and a few minutes later the match was over.
Sitak and Sharan's fifth set match was of a different kind, they lost the first two sets and for the second match in a row came back from that treacherous position to win.
"We had chances in the beginning of the set, but then they got that break and were confident and really fresh still, because it's the beginning of the match," Sitak said.
"In the second set we got into a rhythm and figured out what we needed to do and at 6-5 we had them at 15-40 on Matkowski's serve, so there were two set points right there and unfortunately he hit two big serves to bet get out of it and in the tiebreak they made a few more returns than us.
"After the second set, we said let's keep fighting, keep pushing and looking for those opportunities. If we win the third set and it goes the distance again the other guys might get tired.
"In the middle of the fourth set I was feeling tired myself and I kept telling myself that if I'm feeling like that, the other guys must be exhausted, so let's keep pushing and we'll get an opportunity.
"That's exactly what happened and we got one break in each set."
In the quarterfinals Sitak and Sharan will play against Jack Sock and Mike Bryan on show court No 3, while Venus and Klaasen are up against local hope Jamie Murray and Bruno Soares from Brazil in a match that will be played on centre court.
