PSG, who have a two-point advantage over second-placed Montpellier at the top of the table, needed stoppage-time strikes to clinch a 2-1 victory at Dijon and a 2-2 draw at Caen in their two last league games.
Carlo Ancelotti's side then suffered their first defeat in four months when they lost 3-1 at home to Olympique Lyon in the French Cup quarter-finals on Wednesday, failing to recover from a one-goal deficit despite a strong second-half showing.
"We managed to score in the last minutes of several games but we should have this will to win in the whole match," Alex told a news conference ahead of Sunday's home league encounter with Girondins Bordeaux.
"We have to play well and be aggressive for 90 minutes. If we showed the same will to win as we did in the second half yesterday I think we would not lose another game," added the defender who joined the club from Chelsea in January.
"What we did makes me ashamed," coach Didier Deschamps told reporters after amateurs Quevilly won 3-2 in extra-time to inflict a seventh consecutive defeat in all competitions on his side.
Marseille, who host Bayern Munich in a Champions League quarter-final first leg on Wednesday, travel to relegation-threatened Nice for a Cote d'Azur derby on Saturday.
"It won't be easy. The atmosphere will be particularly hot," said Deschamps.
