Meanwhile, O'Sullivan's hopes of a fourth title this season may come to a premature end at the hands of Maguire, who has beaten him on the last two occasions they have met.
In the men's draw, top-seeded Nikolay Davydenko, who is bidding for his fourth title of the season, reached the last eight with a 6-1 6-1 win over Spain's Guillermo Garcia-Lopez.
But Davydenko, who also won the Malaysian Open at the beginning of October, has put himself in a great position to take one of the three other available places with his fourth title of the season.
The win was Sam's fourth of the season, and with his title rival James Westmoreland finishing second in the race, he needs only a fifth-place finish in the final two rounds to clinch the championship.
The 38-year-old Irishman has suffered some near misses this season as he strove to add the fourth title to his career tally.
Strachan quit his post as Celtic manager at the end of last season after failing to win a fourth successive Scottish championship title.
Nadal won his fifth consecutive Monte Carlo title last Sunday, his fourth of the season, and has only lost four times on the surface since 2005.
Strachan, who missed out on a fourth straight Scottish title on the final day of the league season last weekend, took a bow and received a warm round of applause from the Celtic fans before the charity match.
United, who are seeking a fourth consecutive Premier League title, only have two games remaining this season, at Sunderland on 2 May and when Stoke visit Old Trafford a week later.
She swept all the major events last season, winning her first Grand Prix Final title, her fourth European championship and, in her crowning achievement, that world title.
But without the injured Wayne Rooney, the visiting team lacked a cutting edge and with four games of their league season left United must now be viewed as unlikely to end the campaign with their fourth consecutive Premier League title.
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